diff --git a/core/core.services.yml b/core/core.services.yml
index 9dca742..b40eddd 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -419,3 +419,8 @@ services:
   token:
     class: Drupal\Core\Utility\Token
     arguments: ['@module_handler']
+  theme.registry:
+    class: Drupal\Core\Theme\Registry
+    arguments: ['@cache.cache', '@module_handler']
+    tags:
+      - { name: needs_destruction }
diff --git a/core/includes/common.inc b/core/includes/common.inc
index 4668a90..9e44983 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -4622,8 +4622,6 @@ function _drupal_bootstrap_full($skip = FALSE) {
   if (!defined('MAINTENANCE_MODE') || MAINTENANCE_MODE != 'update') {
     // Prior to invoking hook_init(), initialize the theme (potentially a custom
     // one for this page), so that:
-    // - Modules with hook_init() implementations that call theme() or
-    //   theme_get_registry() don't initialize the incorrect theme.
     // - The theme can have hook_*_alter() implementations affect page building
     //   (e.g., hook_form_alter(), hook_node_view_alter(), hook_page_alter()),
     //   ahead of when rendering starts.
diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index 32c0b49..5a3e437 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -381,6 +381,12 @@ function install_begin_request(&$install_state) {
     // implementation here.
     $container->register('lock', 'Drupal\Core\Lock\NullLockBackend');
 
+    $container
+      ->register('theme.registry', 'Drupal\Core\Theme\Registry')
+      ->addArgument(new Reference('cache.cache'))
+      ->addArgument(new Reference('module_handler'))
+      ->addTag('needs_destruction');
+
     // Register a module handler for managing enabled modules.
     $container
       ->register('module_handler', 'Drupal\Core\Extension\ModuleHandler');
diff --git a/core/includes/module.inc b/core/includes/module.inc
index bc91311..8840ecb 100644
--- a/core/includes/module.inc
+++ b/core/includes/module.inc
@@ -362,7 +362,7 @@ function module_enable($module_list, $enable_dependencies = TRUE) {
       // Refresh the schema to include it.
       drupal_get_schema(NULL, TRUE);
       // Update the theme registry to include it.
-      drupal_theme_rebuild();
+      Drupal::service('theme.registry')->reset();
 
       // Allow modules to react prior to the installation of a module.
       module_invoke_all('modules_preinstall', array($module));
@@ -517,7 +517,7 @@ function module_disable($module_list, $disable_dependents = TRUE) {
     drupal_container()->get('kernel')->updateModules($enabled, $enabled);
 
     // Update the theme registry to remove the newly-disabled module.
-    drupal_theme_rebuild();
+    Drupal::service('theme.registry')->reset();
   }
 }
 
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 0d0e739..acafc43 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -11,6 +11,7 @@
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Config\Config;
 use Drupal\Core\Template\Attribute;
+use Drupal\Core\Theme\Registry;
 use Drupal\Core\Utility\ThemeRegistry;
 use Drupal\Core\Theme\ThemeSettings;
 
@@ -138,10 +139,8 @@ function drupal_theme_initialize() {
  *    the same information as the $theme object. It should be in
  *    'oldest first' order, meaning the top level of the chain will
  *    be first.
- * @param $registry_callback
- *   The callback to invoke to set the theme registry.
  */
-function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callback = '_theme_load_registry') {
+function _drupal_theme_initialize($theme, $base_theme = array()) {
   global $theme_info, $base_theme_info, $theme_engine, $theme_path;
   $theme_info = $theme;
   $base_theme_info = $base_theme;
@@ -268,424 +267,9 @@ function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callb
     }
   }
   // Load twig as secondary always available engine.
-  // @todo Make twig the default engine and remove this. This is required
-  // because (by design) the theme system doesn't allow modules to register more
-  // than one type of extension. We need a temporary backwards compatibility
-  // layer to allow us to perform core-wide .tpl.php to .html.twig conversion.
+  // @todo Replace this hack with generic $theme->engine loading (including base
+  //   themes).
   include_once DRUPAL_ROOT . '/core/themes/engines/twig/twig.engine';
-
-  if (isset($registry_callback)) {
-    _theme_registry_callback($registry_callback, array($theme, $base_theme, $theme_engine));
-  }
-}
-
-/**
- * Gets the theme registry.
- *
- * @param bool $complete
- *   Optional boolean to indicate whether to return the complete theme registry
- *   array or an instance of the Drupal\Core\Utility\ThemeRegistry class.
- *   If TRUE, the complete theme registry array will be returned. This is useful
- *   if you want to foreach over the whole registry, use array_* functions or
- *   inspect it in a debugger. If FALSE, an instance of the
- *   Drupal\Core\Utility\ThemeRegistry class will be returned, this provides an
- *   ArrayObject which allows it to be accessed with array syntax and isset(),
- *   and should be more lightweight than the full registry. Defaults to TRUE.
- *
- * @return
- *   The complete theme registry array, or an instance of the
- *   Drupal\Core\Utility\ThemeRegistry class.
- */
-function theme_get_registry($complete = TRUE) {
-  // Use the advanced drupal_static() pattern, since this is called very often.
-  static $drupal_static_fast;
-  if (!isset($drupal_static_fast)) {
-    $drupal_static_fast['registry'] = &drupal_static('theme_get_registry');
-  }
-  $theme_registry = &$drupal_static_fast['registry'];
-
-  // Initialize the theme, if this is called early in the bootstrap, or after
-  // static variables have been reset.
-  if (!is_array($theme_registry)) {
-    drupal_theme_initialize();
-    $theme_registry = array();
-  }
-
-  $key = (int) $complete;
-
-  if (!isset($theme_registry[$key])) {
-    list($callback, $arguments) = _theme_registry_callback();
-    if (!$complete) {
-      $arguments[] = FALSE;
-    }
-    $theme_registry[$key] = call_user_func_array($callback, $arguments);
-  }
-
-  return $theme_registry[$key];
-}
-
-/**
- * Sets the callback that will be used by theme_get_registry().
- *
- * @param $callback
- *   The name of the callback function.
- * @param $arguments
- *   The arguments to pass to the function.
- */
-function _theme_registry_callback($callback = NULL, array $arguments = array()) {
-  static $stored;
-  if (isset($callback)) {
-    $stored = array($callback, $arguments);
-  }
-  return $stored;
-}
-
-/**
- * Gets the theme_registry cache; if it doesn't exist, builds it.
- *
- * @param $theme
- *   The loaded $theme object as returned by list_themes().
- * @param $base_theme
- *   An array of loaded $theme objects representing the ancestor themes in
- *   oldest first order.
- * @param $theme_engine
- *   The name of the theme engine.
- * @param $complete
- *   Whether to load the complete theme registry or an instance of the
- *   Drupal\Core\Utility\ThemeRegistry class.
- *
- * @return
- *   The theme registry array, or an instance of the
- *   Drupal\Core\Utility\ThemeRegistry class.
- */
-function _theme_load_registry($theme, $base_theme = NULL, $theme_engine = NULL, $complete = TRUE) {
-  if ($complete) {
-    // Check the theme registry cache; if it exists, use it.
-    $cached = cache()->get("theme_registry:$theme->name");
-    if (isset($cached->data)) {
-      $registry = $cached->data;
-    }
-    else {
-      // If not, build one and cache it.
-      $registry = _theme_build_registry($theme, $base_theme, $theme_engine);
-      // Only persist this registry if all modules are loaded. This assures a
-      // complete set of theme hooks.
-      if (drupal_container()->get('module_handler')->isLoaded()) {
-        _theme_save_registry($theme, $registry);
-      }
-    }
-    return $registry;
-  }
-  else {
-    return new ThemeRegistry('theme_registry:runtime:' . $theme->name, 'cache', array('theme_registry' => TRUE), drupal_container()->get('module_handler')->isLoaded());
-  }
-}
-
-/**
- * Writes the theme_registry cache into the database.
- */
-function _theme_save_registry($theme, $registry) {
-  cache()->set("theme_registry:$theme->name", $registry, CacheBackendInterface::CACHE_PERMANENT, array('theme_registry' => TRUE));
-}
-
-/**
- * Forces the system to rebuild the theme registry.
- *
- * This function should be called when modules are added to the system, or when
- * a dynamic system needs to add more theme hooks.
- */
-function drupal_theme_rebuild() {
-  drupal_static_reset('theme_get_registry');
-  cache()->invalidateTags(array('theme_registry' => TRUE));
-}
-
-/**
- * Process a single implementation of hook_theme().
- *
- * @param $cache
- *   The theme registry that will eventually be cached; It is an associative
- *   array keyed by theme hooks, whose values are associative arrays describing
- *   the hook:
- *   - 'type': The passed-in $type.
- *   - 'theme path': The passed-in $path.
- *   - 'function': The name of the function generating output for this theme
- *     hook. Either defined explicitly in hook_theme() or, if neither
- *     'function' nor 'template' is defined, then the default theme function
- *     name is used. The default theme function name is the theme hook prefixed
- *     by either 'theme_' for modules or '$name_' for everything else. If
- *     'function' is defined, 'template' is not used.
- *   - 'template': The filename of the template generating output for this
- *     theme hook. The template is in the directory defined by the 'path' key of
- *     hook_theme() or defaults to "$path/templates".
- *   - 'variables': The variables for this theme hook as defined in
- *     hook_theme(). If there is more than one implementation and 'variables'
- *     is not specified in a later one, then the previous definition is kept.
- *   - 'render element': The renderable element for this theme hook as defined
- *     in hook_theme(). If there is more than one implementation and
- *     'render element' is not specified in a later one, then the previous
- *     definition is kept.
- *   - 'preprocess functions': See theme() for detailed documentation.
- *   - 'process functions': See theme() for detailed documentation.
- * @param $name
- *   The name of the module, theme engine, base theme engine, theme or base
- *   theme implementing hook_theme().
- * @param $type
- *   One of 'module', 'theme_engine', 'base_theme_engine', 'theme', or
- *   'base_theme'. Unlike regular hooks that can only be implemented by modules,
- *   each of these can implement hook_theme(). _theme_process_registry() is
- *   called in aforementioned order and new entries override older ones. For
- *   example, if a theme hook is both defined by a module and a theme, then the
- *   definition in the theme will be used.
- * @param $theme
- *   The loaded $theme object as returned from list_themes().
- * @param $path
- *   The directory where $name is. For example, modules/system or
- *   themes/bartik.
- *
- * @see theme()
- * @see hook_theme()
- * @see list_themes()
- */
-function _theme_process_registry(&$cache, $name, $type, $theme, $path) {
-  $result = array();
-
-  // Processor functions work in two distinct phases with the process
-  // functions always being executed after the preprocess functions.
-  $variable_process_phases = array(
-    'preprocess functions' => 'preprocess',
-    'process functions'    => 'process',
-  );
-
-  $hook_defaults = array(
-    'variables' => TRUE,
-    'render element' => TRUE,
-    'pattern' => TRUE,
-    'base hook' => TRUE,
-  );
-
-  // Invoke the hook_theme() implementation, process what is returned, and
-  // merge it into $cache.
-  $function = $name . '_theme';
-  if (function_exists($function)) {
-    $result = $function($cache, $type, $theme, $path);
-    foreach ($result as $hook => $info) {
-      // When a theme or engine overrides a module's theme function
-      // $result[$hook] will only contain key/value pairs for information being
-      // overridden.  Pull the rest of the information from what was defined by
-      // an earlier hook.
-
-      // Fill in the type and path of the module, theme, or engine that
-      // implements this theme function.
-      $result[$hook]['type'] = $type;
-      $result[$hook]['theme path'] = $path;
-
-      // If function and file are omitted, default to standard naming
-      // conventions.
-      if (!isset($info['template']) && !isset($info['function'])) {
-        $result[$hook]['function'] = ($type == 'module' ? 'theme_' : $name . '_') . $hook;
-      }
-
-      if (isset($cache[$hook]['includes'])) {
-        $result[$hook]['includes'] = $cache[$hook]['includes'];
-      }
-
-      // If the theme implementation defines a file, then also use the path
-      // that it defined. Otherwise use the default path. This allows
-      // system.module to declare theme functions on behalf of core .include
-      // files.
-      if (isset($info['file'])) {
-        $include_file = isset($info['path']) ? $info['path'] : $path;
-        $include_file .= '/' . $info['file'];
-        include_once DRUPAL_ROOT . '/' . $include_file;
-        $result[$hook]['includes'][] = $include_file;
-      }
-
-      // If the default keys are not set, use the default values registered
-      // by the module.
-      if (isset($cache[$hook])) {
-        $result[$hook] += array_intersect_key($cache[$hook], $hook_defaults);
-      }
-
-      // The following apply only to theming hooks implemented as templates.
-      if (isset($info['template'])) {
-        // Prepend the current theming path when none is set.
-        if (!isset($info['path'])) {
-          $result[$hook]['template'] = $path . '/templates/' . $info['template'];
-        }
-        if ($type == 'module' || $type == 'theme_engine') {
-          // Add two render engines for modules and theme engines.
-          // @todo Remove and make twig the default engine.
-          $render_engines = array(
-            '.html.twig' => 'twig',
-            '.tpl.php' => 'phptemplate'
-          );
-
-          // Find the best engine for this template.
-          foreach ($render_engines as $extension => $engine) {
-            // Render the output using the template file.
-            $template_file = $result[$hook]['template'] . $extension;
-            if (isset($info['path'])) {
-              $template_file = $info['path'] . '/' . $template_file;
-            }
-            if (file_exists($template_file)) {
-              $result[$hook]['template_file'] = $template_file;
-              $result[$hook]['engine'] = $engine;
-              break;
-            }
-          }
-        }
-      }
-
-      // Allow variable processors for all theming hooks, whether the hook is
-      // implemented as a template or as a function.
-      foreach ($variable_process_phases as $phase_key => $phase) {
-        // Check for existing variable processors. Ensure arrayness.
-        if (!isset($info[$phase_key]) || !is_array($info[$phase_key])) {
-          $info[$phase_key] = array();
-          $prefixes = array();
-          if ($type == 'module') {
-            // Default variable processor prefix.
-            $prefixes[] = 'template';
-            // Add all modules so they can intervene with their own variable
-            // processors. This allows them to provide variable processors even
-            // if they are not the owner of the current hook.
-            $prefixes = array_merge($prefixes, array_keys(drupal_container()->get('module_handler')->getModuleList()));
-          }
-          elseif ($type == 'theme_engine' || $type == 'base_theme_engine') {
-            // Theme engines get an extra set that come before the normally
-            // named variable processors.
-            $prefixes[] = $name . '_engine';
-            // The theme engine registers on behalf of the theme using the
-            // theme's name.
-            $prefixes[] = $theme;
-          }
-          else {
-            // This applies when the theme manually registers their own variable
-            // processors.
-            $prefixes[] = $name;
-          }
-          foreach ($prefixes as $prefix) {
-            // Only use non-hook-specific variable processors for theming hooks
-            // implemented as templates. See theme().
-            if (isset($info['template']) && function_exists($prefix . '_' . $phase)) {
-              $info[$phase_key][] = $prefix . '_' . $phase;
-            }
-            if (function_exists($prefix . '_' . $phase . '_' . $hook)) {
-              $info[$phase_key][] = $prefix . '_' . $phase . '_' . $hook;
-            }
-          }
-        }
-        // Check for the override flag and prevent the cached variable
-        // processors from being used. This allows themes or theme engines to
-        // remove variable processors set earlier in the registry build.
-        if (!empty($info['override ' . $phase_key])) {
-          // Flag not needed inside the registry.
-          unset($result[$hook]['override ' . $phase_key]);
-        }
-        elseif (isset($cache[$hook][$phase_key]) && is_array($cache[$hook][$phase_key])) {
-          $info[$phase_key] = array_merge($cache[$hook][$phase_key], $info[$phase_key]);
-        }
-        $result[$hook][$phase_key] = $info[$phase_key];
-      }
-    }
-
-    // Merge the newly created theme hooks into the existing cache.
-    $cache = $result + $cache;
-  }
-
-  // Let themes have variable processors even if they didn't register a
-  // template.
-  if ($type == 'theme' || $type == 'base_theme') {
-    foreach ($cache as $hook => $info) {
-      // Check only if not registered by the theme or engine.
-      if (empty($result[$hook])) {
-        foreach ($variable_process_phases as $phase_key => $phase) {
-          if (!isset($info[$phase_key])) {
-            $cache[$hook][$phase_key] = array();
-          }
-          // Only use non-hook-specific variable processors for theming hooks
-          // implemented as templates. See theme().
-          if (isset($info['template']) && function_exists($name . '_' . $phase)) {
-            $cache[$hook][$phase_key][] = $name . '_' . $phase;
-          }
-          if (function_exists($name . '_' . $phase . '_' . $hook)) {
-            $cache[$hook][$phase_key][] = $name . '_' . $phase . '_' . $hook;
-            $cache[$hook]['theme path'] = $path;
-          }
-          // Ensure uniqueness.
-          $cache[$hook][$phase_key] = array_unique($cache[$hook][$phase_key]);
-        }
-      }
-    }
-  }
-}
-
-/**
- * Builds the theme registry cache.
- *
- * @param $theme
- *   The loaded $theme object as returned by list_themes().
- * @param $base_theme
- *   An array of loaded $theme objects representing the ancestor themes in
- *   oldest first order.
- * @param $theme_engine
- *   The name of the theme engine.
- */
-function _theme_build_registry($theme, $base_theme, $theme_engine) {
-  $cache = array();
-  // First, process the theme hooks advertised by modules. This will
-  // serve as the basic registry. Since the list of enabled modules is the same
-  // regardless of the theme used, this is cached in its own entry to save
-  // building it for every theme.
-  if ($cached = cache()->get('theme_registry:build:modules')) {
-    $cache = $cached->data;
-  }
-  else {
-    foreach (module_implements('theme') as $module) {
-      _theme_process_registry($cache, $module, 'module', $module, drupal_get_path('module', $module));
-    }
-    // Only cache this registry if all modules are loaded.
-    if (drupal_container()->get('module_handler')->isLoaded()) {
-      cache()->set("theme_registry:build:modules", $cache, CacheBackendInterface::CACHE_PERMANENT, array('theme_registry' => TRUE));
-    }
-  }
-
-  // Process each base theme.
-  foreach ($base_theme as $base) {
-    // If the base theme uses a theme engine, process its hooks.
-    $base_path = dirname($base->filename);
-    if ($theme_engine) {
-      _theme_process_registry($cache, $theme_engine, 'base_theme_engine', $base->name, $base_path);
-    }
-    _theme_process_registry($cache, $base->name, 'base_theme', $base->name, $base_path);
-  }
-
-  // And then the same thing, but for the theme.
-  if ($theme_engine) {
-    _theme_process_registry($cache, $theme_engine, 'theme_engine', $theme->name, dirname($theme->filename));
-  }
-
-  if ($theme_engine == 'phptemplate') {
-    // Check for Twig templates if this is a PHPTemplate theme.
-    // @todo Remove this once all core themes are converted to Twig.
-    _theme_process_registry($cache, 'twig', 'theme_engine', $theme->name, dirname($theme->filename));
-  }
-
-  // Finally, hooks provided by the theme itself.
-  _theme_process_registry($cache, $theme->name, 'theme', $theme->name, dirname($theme->filename));
-
-  // Let modules alter the registry.
-  drupal_alter('theme_registry', $cache);
-
-  // Optimize the registry to not have empty arrays for functions.
-  foreach ($cache as $hook => $info) {
-    foreach (array('preprocess functions', 'process functions') as $phase) {
-      if (empty($info[$phase])) {
-        unset($cache[$hook][$phase]);
-      }
-    }
-  }
-  return $cache;
 }
 
 /**
@@ -868,6 +452,7 @@ function drupal_find_base_themes($themes, $key, $used_keys = array()) {
  * executed (if they exist), in the following order (note that in the following
  * list, HOOK indicates the theme hook name, MODULE indicates a module name,
  * THEME indicates a theme name, and ENGINE indicates a theme engine name):
+ * @todo Needs update for declarative *_preprocess_HOOK().
  * - template_preprocess(&$variables, $hook): Creates a default set of
  *   variables for all theme hooks with template implementations.
  * - template_preprocess_HOOK(&$variables): Should be implemented by the module
@@ -966,12 +551,12 @@ function theme($hook, $variables = array()) {
   static $default_attributes;
   // If called before all modules are loaded, we do not necessarily have a full
   // theme registry to work with, and therefore cannot process the theme
-  // request properly. See also _theme_load_registry().
+  // request properly.
   if (!drupal_container()->get('module_handler')->isLoaded() && !defined('MAINTENANCE_MODE')) {
     throw new Exception(t('theme() may not be called until all modules are loaded.'));
   }
 
-  $hooks = theme_get_registry(FALSE);
+  $hooks = \Drupal::service('theme.registry')->getRuntime();
 
   // If an array of hook candidates were passed, use the first one that has an
   // implementation.
@@ -1066,22 +651,20 @@ function theme($hook, $variables = array()) {
         include_once DRUPAL_ROOT . '/' . $include_file;
       }
     }
-    if (isset($base_hook_info['preprocess functions']) || isset($base_hook_info['process functions'])) {
+    if (isset($base_hook_info['preprocess']) || isset($base_hook_info['process'])) {
       $variables['theme_hook_suggestion'] = $hook;
       $hook = $base_hook;
       $info = $base_hook_info;
     }
   }
-  if (isset($info['preprocess functions']) || isset($info['process functions'])) {
+  if (isset($info['preprocess']) || isset($info['process'])) {
     $variables['theme_hook_suggestions'] = array();
-    foreach (array('preprocess functions', 'process functions') as $phase) {
+    foreach (array('preprocess', 'process') as $phase) {
       if (!empty($info[$phase])) {
         foreach ($info[$phase] as $processor_function) {
-          if (function_exists($processor_function)) {
-            // We don't want a poorly behaved process function changing $hook.
-            $hook_clone = $hook;
-            $processor_function($variables, $hook_clone);
-          }
+          // We don't want a poorly behaved process function changing $hook.
+          $hook_clone = $hook;
+          $processor_function($variables, $hook_clone);
         }
       }
     }
@@ -1106,6 +689,11 @@ function theme($hook, $variables = array()) {
     }
     foreach (array_reverse($suggestions) as $suggestion) {
       if (isset($hooks[$suggestion])) {
+        // A theme hook suggestion is essentially the reverse of a base hook
+        // definition; instead of finding the generic parent, we find a more
+        // specific child. The registry ensures to merge the base hook info into
+        // the suggestion's info.
+        // @see \Drupal\Core\Theme\Registry::build()
         $info = $hooks[$suggestion];
         break;
       }
@@ -1114,12 +702,10 @@ function theme($hook, $variables = array()) {
 
   // Generate the output using either a function or a template.
   $output = '';
-  if (isset($info['function'])) {
-    if (function_exists($info['function'])) {
-      $output = $info['function']($variables);
-    }
+  if (!isset($info['template']) && isset($info['function'])) {
+    $output = $info['function']($variables);
   }
-  else {
+  elseif (isset($info['template'])) {
     // Default render function and extension.
     $render_function = 'theme_render_template';
     $extension = '.tpl.php';
@@ -1145,21 +731,6 @@ function theme($hook, $variables = array()) {
       }
     }
 
-    // In some cases, a template implementation may not have had
-    // template_preprocess() run (for example, if the default implementation is
-    // a function, but a template overrides that default implementation). In
-    // these cases, a template should still be able to expect to have access to
-    // the variables provided by template_preprocess(), so we add them here if
-    // they don't already exist. We don't want the overhead of running
-    // template_preprocess() twice, so we use the 'directory' variable to
-    // determine if it has already run, which while not completely intuitive,
-    // is reasonably safe, and allows us to save on the overhead of adding some
-    // new variable to track that.
-    if (!isset($variables['directory'])) {
-      $default_template_variables = array();
-      template_preprocess($default_template_variables, $hook);
-      $variables += $default_template_variables;
-    }
     if (!isset($default_attributes)) {
       $default_attributes = new Attribute();
     }
@@ -1176,15 +747,12 @@ function theme($hook, $variables = array()) {
     }
 
     // Render the output using the template file.
-    $template_file = $info['template'] . $extension;
-    if (isset($info['path'])) {
-      $template_file = $info['path'] . '/' . $template_file;
-    }
-
-    // Modules can override this.
     if (isset($info['template_file'])) {
       $template_file = $info['template_file'];
     }
+    else {
+      $template_file = $info['path'] . '/' . $info['template'] . $extension;
+    }
 
     $output = $render_function($template_file, $variables);
   }
@@ -1248,10 +816,8 @@ function drupal_find_theme_functions($cache, $prefixes) {
         if ($matches) {
           foreach ($matches as $match) {
             $new_hook = substr($match, strlen($prefix) + 1);
-            $arg_name = isset($info['variables']) ? 'variables' : 'render element';
             $implementations[$new_hook] = array(
               'function' => $match,
-              $arg_name => $info[$arg_name],
               'base hook' => $hook,
             );
           }
@@ -1362,11 +928,9 @@ function drupal_find_theme_templates($cache, $extension, $path) {
           $file = str_replace($extension, '', $file);
           // Put the underscores back in for the hook name and register this
           // pattern.
-          $arg_name = isset($info['variables']) ? 'variables' : 'render element';
           $implementations[strtr($file, '-', '_')] = array(
             'template' => $file,
             'path' => dirname($files[$match]->uri),
-            $arg_name => $info[$arg_name],
             'base hook' => $hook,
           );
         }
@@ -1571,7 +1135,7 @@ function theme_enable($theme_list) {
 
   list_themes(TRUE);
   menu_router_rebuild();
-  drupal_theme_rebuild();
+  Drupal::service('theme.registry')->reset();
 
   // Invoke hook_themes_enabled() after the themes have been enabled.
   module_invoke_all('themes_enabled', $theme_list);
@@ -1606,7 +1170,7 @@ function theme_disable($theme_list) {
 
   list_themes(TRUE);
   menu_router_rebuild();
-  drupal_theme_rebuild();
+  Drupal::service('theme.registry')->reset();
 
   // Invoke hook_themes_disabled after the themes have been disabled.
   module_invoke_all('themes_disabled', $theme_list);
@@ -3181,18 +2745,24 @@ function drupal_common_theme() {
     // From theme.inc.
     'html' => array(
       'render element' => 'page',
+      'preprocess' => array('template_preprocess_html'),
+      'process' => array('template_process_html'),
       'template' => 'html',
     ),
     'page' => array(
       'render element' => 'page',
+      'preprocess' => array('template_preprocess_page'),
+      'process' => array('template_process_page'),
       'template' => 'page',
     ),
     'region' => array(
       'render element' => 'elements',
+      'preprocess' => array('template_preprocess_region'),
       'template' => 'region',
     ),
     'datetime' => array(
       'variables' => array('timestamp' => NULL, 'text' => NULL, 'attributes' => array(), 'html' => FALSE),
+      'preprocess' => array('template_preprocess_datetime'),
       'template' => 'datetime',
     ),
     'status_messages' => array(
@@ -3239,6 +2809,7 @@ function drupal_common_theme() {
     ),
     'item_list' => array(
       'variables' => array('items' => array(), 'title' => '', 'type' => 'ul', 'attributes' => array()),
+      'preprocess' => array('template_preprocess_item_list'),
     ),
     'more_help_link' => array(
       'variables' => array('url' => NULL),
@@ -3261,103 +2832,123 @@ function drupal_common_theme() {
     // From theme.maintenance.inc.
     'maintenance_page' => array(
       'variables' => array('content' => NULL, 'show_messages' => TRUE),
+      'includes' => array('core/includes/theme.maintenance.inc'),
+      'preprocess' => array('template_preprocess_maintenance_page'),
+      'process' => array('template_process_maintenance_page'),
       'template' => 'maintenance-page',
     ),
     'install_page' => array(
       'variables' => array('content' => NULL),
+      'includes' => array('core/includes/theme.maintenance.inc'),
     ),
     'task_list' => array(
       'variables' => array('items' => NULL, 'active' => NULL),
+      'includes' => array('core/includes/theme.maintenance.inc'),
     ),
     'authorize_message' => array(
       'variables' => array('message' => NULL, 'success' => TRUE),
+      'includes' => array('core/includes/theme.maintenance.inc'),
     ),
     'authorize_report' => array(
       'variables' => array('messages' => array()),
+      'includes' => array('core/includes/theme.maintenance.inc'),
     ),
     // From pager.inc.
     'pager' => array(
       'variables' => array('tags' => array(), 'element' => 0, 'parameters' => array(), 'quantity' => 9),
-    ),
-    'pager_first' => array(
-      'variables' => array('text' => NULL, 'element' => 0, 'parameters' => array()),
-    ),
-    'pager_previous' => array(
-      'variables' => array('text' => NULL, 'element' => 0, 'interval' => 1, 'parameters' => array()),
-    ),
-    'pager_next' => array(
-      'variables' => array('text' => NULL, 'element' => 0, 'interval' => 1, 'parameters' => array()),
-    ),
-    'pager_last' => array(
-      'variables' => array('text' => NULL, 'element' => 0, 'parameters' => array()),
+      'includes' => array('core/includes/pager.inc'),
     ),
     'pager_link' => array(
       'variables' => array('text' => NULL, 'page_new' => NULL, 'element' => NULL, 'parameters' => array(), 'attributes' => array()),
+      'includes' => array('core/includes/pager.inc'),
     ),
     // From menu.inc.
     'menu_link' => array(
       'render element' => 'element',
+      'includes' => array('core/includes/menu.inc'),
     ),
     'menu_tree' => array(
       'render element' => 'tree',
+      'preprocess' => array('template_preprocess_menu_tree'),
+      'includes' => array('core/includes/menu.inc'),
     ),
     'menu_local_task' => array(
       'render element' => 'element',
+      'includes' => array('core/includes/menu.inc'),
     ),
     'menu_local_action' => array(
       'render element' => 'element',
+      'includes' => array('core/includes/menu.inc'),
     ),
     'menu_local_tasks' => array(
       'variables' => array('primary' => array(), 'secondary' => array()),
+      'includes' => array('core/includes/menu.inc'),
     ),
     // From form.inc.
     'input' => array(
       'render element' => 'element',
+      'preprocess' => array('template_preprocess_input'),
+      'includes' => array('core/includes/form.inc'),
     ),
     'select' => array(
       'render element' => 'element',
+      'includes' => array('core/includes/form.inc'),
     ),
     'fieldset' => array(
       'render element' => 'element',
+      'includes' => array('core/includes/form.inc'),
     ),
     'details' => array(
       'render element' => 'element',
+      'includes' => array('core/includes/form.inc'),
     ),
     'radios' => array(
       'render element' => 'element',
+      'includes' => array('core/includes/form.inc'),
     ),
     'date' => array(
       'render element' => 'element',
+      'includes' => array('core/includes/form.inc'),
     ),
     'exposed_filters' => array(
       'render element' => 'form',
+      'includes' => array('core/includes/form.inc'),
     ),
     'checkboxes' => array(
       'render element' => 'element',
+      'includes' => array('core/includes/form.inc'),
     ),
     'form' => array(
       'render element' => 'element',
+      'includes' => array('core/includes/form.inc'),
     ),
     'textarea' => array(
       'render element' => 'element',
+      'includes' => array('core/includes/form.inc'),
     ),
     'tableselect' => array(
       'render element' => 'element',
+      'includes' => array('core/includes/form.inc'),
     ),
     'form_element' => array(
       'render element' => 'element',
+      'includes' => array('core/includes/form.inc'),
     ),
     'form_required_marker' => array(
       'render element' => 'element',
+      'includes' => array('core/includes/form.inc'),
     ),
     'form_element_label' => array(
       'render element' => 'element',
+      'includes' => array('core/includes/form.inc'),
     ),
     'vertical_tabs' => array(
       'render element' => 'element',
+      'includes' => array('core/includes/form.inc'),
     ),
     'container' => array(
       'render element' => 'element',
+      'includes' => array('core/includes/form.inc'),
     ),
   );
 }
diff --git a/core/includes/theme.maintenance.inc b/core/includes/theme.maintenance.inc
index 9215957..b770d09 100644
--- a/core/includes/theme.maintenance.inc
+++ b/core/includes/theme.maintenance.inc
@@ -90,7 +90,10 @@ function _drupal_maintenance_theme() {
     $base_theme[] = $new_base_theme = $themes[$themes[$ancestor]->base_theme];
     $ancestor = $themes[$ancestor]->base_theme;
   }
-  _drupal_theme_initialize($themes[$theme], array_reverse($base_theme), '_theme_load_offline_registry');
+  _drupal_theme_initialize($themes[$theme], array_reverse($base_theme));
+  // Prime the theme registry.
+  // @todo Remove global theme variables.
+  Drupal::service('theme.registry');
 
   // These are usually added from system_init() -except maintenance.css.
   // When the database is inactive it's not called so we add it here.
@@ -102,13 +105,6 @@ function _drupal_maintenance_theme() {
 }
 
 /**
- * Builds the registry when the site needs to bypass any database calls.
- */
-function _theme_load_offline_registry($theme, $base_theme = NULL, $theme_engine = NULL) {
-  return _theme_build_registry($theme, $base_theme, $theme_engine);
-}
-
-/**
  * Returns HTML for a list of maintenance tasks to perform.
  *
  * @param $variables
diff --git a/core/includes/update.inc b/core/includes/update.inc
index a5feb50..764b23d 100644
--- a/core/includes/update.inc
+++ b/core/includes/update.inc
@@ -429,6 +429,9 @@ function update_prepare_d8_bootstrap() {
   new Settings($settings);
   $kernel = new DrupalKernel('update', FALSE, drupal_classloader(), FALSE);
   $kernel->boot();
+
+  // Remove the theme_registry cache entry from D7.
+  Drupal::cache('cache')->deleteAll();
 }
 
 /**
diff --git a/core/lib/Drupal/Core/EventSubscriber/LegacyRequestSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/LegacyRequestSubscriber.php
index 242f935..0d9f76d 100644
--- a/core/lib/Drupal/Core/EventSubscriber/LegacyRequestSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/LegacyRequestSubscriber.php
@@ -29,8 +29,6 @@ public function onKernelRequestLegacy(GetResponseEvent $event) {
     if ($event->getRequestType() == HttpKernelInterface::MASTER_REQUEST) {
       // Prior to invoking hook_init(), initialize the theme (potentially a
       // custom one for this page), so that:
-      // - Modules with hook_init() implementations that call theme() or
-      //   theme_get_registry() don't initialize the incorrect theme.
       // - The theme can have hook_*_alter() implementations affect page
       //   building (e.g., hook_form_alter(), hook_node_view_alter(),
       //   hook_page_alter()), ahead of when rendering starts.
diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
new file mode 100644
index 0000000..7146f47
--- /dev/null
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -0,0 +1,717 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Theme\Registry.
+ */
+
+namespace Drupal\Core\Theme;
+
+use Drupal\Component\Utility\NestedArray;
+use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\DestructableInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Utility\ThemeRegistry;
+
+/**
+ * Defines the theme registry service.
+ *
+ * @todo Replace local $registry variables in methods with $this->registry.
+ */
+class Registry implements DestructableInterface {
+
+  /**
+   * The theme object representing the active theme for this registry.
+   *
+   * @var object
+   */
+  protected $theme;
+
+  /**
+   * An array of base theme objects.
+   *
+   * @var array
+   */
+  protected $baseThemes;
+
+  /**
+   * The name of the theme engine of $theme.
+   *
+   * @var string
+   */
+  protected $engine;
+
+  /**
+   * The complete theme registry.
+   *
+   * @var array
+   *   An associative array keyed by theme hook names, whose values are
+   *   associative arrays containing the aggregated hook definition:
+   *   - type: The type of the extension the original theme hook originates
+   *     from; e.g., 'module' for theme hook 'node' of Node module.
+   *   - name: The name of the extension the original theme hook originates
+   *     from; e.g., 'node' for theme hook 'node' of Node module.
+   *   - theme path: The effective path_to_theme() during theme(), available as
+   *     'directory' variable in templates.
+   *     @todo Remove 'theme path', it's useless... or fix it: For theme
+   *       functions, it should point to the respective theme. For templates,
+   *       it should point to the directory that contains the template.
+   *   - includes: (optional) An array of include files to load when the theme
+   *     hook is executed by theme().
+   *   - file: (optional) A filename to add to 'includes', either prefixed with
+   *     the value of 'path', or the path of the extension implementing
+   *     hook_theme().
+   *   In case of a theme base hook, one of the following:
+   *   - variables: An associative array whose keys are variable names and whose
+   *     values are default values of the variables to use for this theme hook.
+   *   - render element: A string denoting the name of the variable name, in
+   *     which the render element for this theme hook is provided.
+   *   In case of a theme template file:
+   *   - path: The path to the template file to use. Defaults to the
+   *     subdirectory 'templates' of the path of the extension implementing
+   *     hook_theme(); e.g., 'core/modules/node/templates' for Node module.
+   *   - template: The basename of the template file to use, without extension
+   *     (as the extension is specific to the theme engine). The template file
+   *     is in the directory defined by 'path'.
+   *   - template_file: A full path and file name to a template file to use.
+   *     Allows any extension to override the effective template file.
+   *   - engine: The theme engine to use for the template file.
+   *   In case of a theme function:
+   *   - function: The function name to call to generate the output.
+   *   For any registered theme hook, including theme hook suggestions:
+   *   - preprocess: An array of theme variable preprocess callbacks to invoke
+   *     before invoking final theme variable processors.
+   *   - process: An array of theme variable process callbacks to invoke
+   *     before invoking the actual theme function or template.
+   */
+  protected $registry;
+
+  /**
+   * The cache backend to use for the complete theme registry data.
+   *
+   * @var \Drupal\Core\Cache\CacheBackendInterface
+   */
+  protected $cache;
+
+  /**
+   * The module handler to use to load modules.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface
+   */
+  protected $moduleHandler;
+
+  /**
+   * The incomplete, runtime theme registry.
+   *
+   * @var \Drupal\Core\Utility\ThemeRegistry
+   */
+  protected $runtimeRegistry;
+
+
+  /**
+   * Constructs a \Drupal\Core\\Theme\Registry object.
+   *
+   * @param \Drupal\Core\Cache\CacheBackendInterface $cache
+   *   The cache backend interface to use for the complete theme registry data.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   The module handler to use to load modules.
+   * @param string $theme_name
+   *   (optional) The name of the theme for which to construct the registry.
+   */
+  public function __construct(CacheBackendInterface $cache, ModuleHandlerInterface $module_handler, $theme_name = NULL) {
+    $this->cache = $cache;
+    $this->moduleHandler = $module_handler;
+    $this->init($theme_name);
+  }
+
+  /**
+   * Initializes a theme with a certain name.
+   *
+   * This function does to much magic, so it should be replaced by another
+   * services which holds the current active theme information.
+   *
+   * @param string $theme_name
+   *   (optional) The name of the theme for which to construct the registry.
+   *
+   * @global object $theme_info
+   *   An object with (at least) the following information:
+   *   - uri: The path to the theme.
+   *   - owner: The name of the theme's base theme.
+   *   - engine: The name of the theme engine to use.
+   * @global array $base_theme_info
+   *   (optional) An array of objects that represent the base themes of $theme,
+   *   each having the same properties as $theme above, ordered by base theme
+   *   hierarchy; i.e., the first element is the root of all themes.
+   * @global string $theme_engine
+   *   The name of the theme engine.
+   *
+   * @todo Remove global $theme_engine, it duplicates ->engine.
+   * @todo Inject ThemeHandler, so as to remove all of these globals,
+   *   in this way:
+   *   - theme engines are dependencies of themes.
+   *   - theme engines are actually treated *identically* to themes. (also:
+   *     remove the /engines subdirectory)
+   *   - base themes are dependencies of themes.
+   *   As a result:
+   *   - $theme->requires == should contain the full stack of dependencies,
+   *     in the correct order.
+   *
+   * @todo Merge this into ::get(), so modules + tests can retrieve the registry
+   *   for a particular $theme_name.
+   */
+  protected function init($theme_name = NULL) {
+    // Unless instantiated for a specific theme, use globals.
+    if (!isset($theme_name)) {
+      // #1: The theme registry might get instantiated before the theme was
+      // initialized. Cope with that.
+      if (!isset($GLOBALS['theme_info'])) {
+        unset($this->runtimeRegistry);
+        unset($this->registry);
+        drupal_theme_initialize();
+      }
+      // #2: The testing framework only cares for the global $theme variable at
+      // this point. Cope with that.
+      if ($GLOBALS['theme'] != $GLOBALS['theme_info']->name) {
+        unset($this->runtimeRegistry);
+        unset($this->registry);
+        drupal_theme_initialize();
+      }
+      $this->theme = $GLOBALS['theme_info'];
+      $this->baseThemes = $GLOBALS['base_theme_info'];
+      $this->engine = $GLOBALS['theme_engine'];
+    }
+    // Instead of the global theme, a specific theme was requested.
+    else {
+      // @see drupal_theme_initialize()
+      $themes = list_themes();
+      $this->theme = $themes[$theme_name];
+
+      // Find all base themes.
+      $this->baseThemes = array();
+      $ancestor = $theme_name;
+      while ($ancestor && isset($themes[$ancestor]->base_theme)) {
+        $ancestor = $themes[$ancestor]->base_theme;
+        $this->baseThemes[] = $themes[$ancestor];
+        if (!empty($themes[$ancestor]->owner)) {
+          include_once DRUPAL_ROOT . '/' . $themes[$ancestor]->owner;
+        }
+      }
+      $this->baseThemes = array_reverse($this->baseThemes);
+
+      // @see _drupal_theme_initialize()
+      if (isset($this->theme->engine)) {
+        $this->engine = $this->theme->engine;
+        include_once DRUPAL_ROOT . '/' . $this->theme->owner;
+        if (function_exists($this->theme->engine . '_init')) {
+          foreach ($this->baseThemes as $base) {
+            call_user_func($this->theme->engine . '_init', $base);
+          }
+          call_user_func($this->theme->engine . '_init', $this->theme);
+        }
+      }
+    }
+  }
+
+  /**
+   * Returns the complete theme registry from cache or rebuilds it.
+   *
+   * @return array
+   *   The complete theme registry data array.
+   *
+   * @see Registry::$registry
+   *
+   * @todo Add a lock for the (re)build operation. It's relatively quick, but
+   *   not necessarily fast enough in case of many parallel requests.
+   */
+  public function get() {
+    if (isset($this->registry)) {
+      return $this->registry;
+    }
+    if ($cache = $this->cache->get('theme_registry:' . $this->theme->name)) {
+      $this->registry = $cache->data;
+    }
+    else {
+      $this->registry = $this->build();
+      // Only persist it if all modules are loaded to ensure it is complete.
+      if ($this->moduleHandler->isLoaded()) {
+        $this->setCache();
+      }
+    }
+    return $this->registry;
+  }
+
+  /**
+   * Returns the incomplete, runtime theme registry.
+   *
+   * @return \Drupal\Core\Utility\ThemeRegistry
+   *   A shared instance of the ThemeRegistry class, provides an ArrayObject
+   *   that allows it to be accessed with array syntax and isset(), and is more
+   *   lightweight than the full registry.
+   */
+  public function getRuntime() {
+    if (!isset($this->runtimeRegistry)) {
+      $this->runtimeRegistry = new ThemeRegistry('theme_registry:runtime:' . $this->theme->name, 'cache', array('theme_registry' => TRUE), $this->moduleHandler->isLoaded());
+    }
+    return $this->runtimeRegistry;
+  }
+
+  /**
+   * Persists the theme registry in the cache backend.
+   */
+  protected function setCache() {
+    $this->cache->set('theme_registry:' . $this->theme->name, $this->registry, CacheBackendInterface::CACHE_PERMANENT, array('theme_registry' => TRUE));
+  }
+
+  /**
+   * Builds the theme registry from scratch.
+   *
+   * Theme hook definitions are collected in the following order:
+   * - Modules
+   * - Base theme engines
+   * - Base themes
+   * - Theme engine
+   * - Theme
+   *
+   * All theme hook definitions are essentially just collated and merged in the
+   * above order. However, various extension-specific default values and
+   * customizations are required; e.g., to record the effective file path for
+   * theme template. Therefore, this method first collects all extensions per
+   * type, and then dispatches the processing for each extension to
+   * processExtension().
+   *
+   * @see hook_theme()
+   *
+   * After completing the collection, modules are allowed to alter it. Lastly,
+   * any derived and incomplete theme hook definitions that are hook suggestions
+   * for base hooks (e.g., 'block__node' for the base hook 'block') need to be
+   * determined based on the full registry and classified as 'base hook'.
+   *
+   * @see theme()
+   * @see hook_theme_registry_alter()
+   */
+  protected function build() {
+    // @todo Replace local $registry variables in methods with $this->registry.
+    $registry = array();
+
+    // hook_theme() implementations of modules are always the same.
+    if ($cache = $this->cache->get('theme_registry:build:modules')) {
+      $registry = $cache->data;
+    }
+    else {
+      foreach ($this->moduleHandler->getImplementations('theme') as $module) {
+        $this->processExtension($registry, 'module', $module, $module, drupal_get_path('module', $module));
+      }
+      // Only persist it if all modules are loaded to ensure it is complete.
+      // @todo Get rid of these checks.
+      if ($this->moduleHandler->isLoaded()) {
+        $this->cache->set("theme_registry:build:modules", $registry, CacheBackendInterface::CACHE_PERMANENT, array('theme_registry' => TRUE), TRUE);
+      }
+    }
+
+    // Process each base theme.
+    foreach ($this->baseThemes as $base) {
+      // If the base theme uses a theme engine, process its hooks.
+      $base_path = dirname($base->uri);
+      if ($this->engine) {
+        $this->processExtension($registry, 'base_theme_engine', $this->engine, $base->name, $base_path);
+      }
+      $this->processExtension($registry, 'base_theme', $base->name, $base->name, $base_path);
+    }
+
+    // Process the theme itself.
+    if ($this->engine) {
+      $this->processExtension($registry, 'theme_engine', $this->engine, $this->theme->name, dirname($this->theme->uri));
+    }
+    if ($this->engine == 'phptemplate') {
+      // Check for Twig templates if this is a PHPTemplate theme.
+      // @todo Remove this once all core themes are converted to Twig.
+      $this->processExtension($registry, 'theme_engine', 'twig', $this->theme->name, dirname($this->theme->uri));
+    }
+    $this->processExtension($registry, 'theme', $this->theme->name, $this->theme->name, dirname($this->theme->uri));
+
+    // Allow modules to alter the complete registry.
+    $this->moduleHandler->alter('theme_registry', $registry);
+
+    $this->registry = $registry;
+    $this->compile();
+    return $this->registry;
+  }
+
+  /**
+   * Process a single implementation of hook_theme().
+   *
+   * @param array $registry
+   *   The theme registry that will eventually be cached.
+   * @param string $type
+   *   One of 'module', 'base_theme_engine', 'base_theme', 'theme_engine', or
+   *   'theme'.
+   * @param string $name
+   *   The name of the extension implementing hook_theme().
+   * @param string $theme_name
+   *   The name of the extension for which theme hooks are currently processed.
+   *   This equals $name for all extension types, except when $type is a theme
+   *   engine, in which case $theme_name and $theme_path are pointing to the
+   *   respective [base] theme the theme engine is associated with.
+   * @param string $theme_path
+   *   The directory of $theme_name; e.g., 'core/modules/node' or
+   *   'themes/bartik'.
+   *
+   * @see theme()
+   * @see hook_theme()
+   */
+  protected function processExtension(&$registry, $type, $name, $theme_name, $theme_path) {
+    $function = $name . '_theme';
+    // Extensions do not necessarily have to implement hook_theme().
+    if (!function_exists($function)) {
+      return;
+    }
+    foreach ($function($registry, $type, $theme_name, $theme_path) as $hook => $info) {
+      // Ensure this hook key exists; it will be set either way.
+      $registry += array($hook => array(
+        // @todo It's not really clear what these are pointing to, whether they
+        //   are used anywhere, and how, and whether they are needed at all.
+        'type' => $type,
+        'name' => $name,
+      ));
+
+      if (isset($info['file'])) {
+        $include_path = isset($info['path']) ? $info['path'] : $theme_path;
+        $info['includes'][] = $include_path . '/' . $info['file'];
+        unset($info['file']);
+      }
+
+      // An actual/original theme hook must define either 'variables' or a
+      // 'render element', in which case we need to assign default values for
+      // 'template' or 'function'.
+      if (isset($info['variables']) || isset($info['render element'])) {
+        // Add an internal build process marker to track that this an actual
+        // theme hook and not a suggestion.
+        $info['exists'] = TRUE;
+
+        // The effective path_to_theme() during theme().
+        $info['theme path'] = $theme_path;
+
+        if (isset($info['template'])) {
+          // Default the template path to the 'templates' directory of the
+          // extension, unless overridden.
+          if (!isset($info['path'])) {
+            $info['path'] = $theme_path . '/templates';
+          }
+          // Find the preferred theme engine for this module template.
+          // @todo Remove this. Simply support multiple theme engines;
+          //   which will simplify the entire processing in the first place.
+          if ($type == 'module' || $type == 'theme_engine') {
+            // Add two render engines for modules and theme engines.
+            $render_engines = array(
+              '.html.twig' => 'twig',
+              '.tpl.php' => 'phptemplate',
+            );
+            // Find the best engine for this template.
+            foreach ($render_engines as $extension => $engine) {
+              // Render the output using the template file.
+              $template_file = $info['path'] . '/' . $info['template'] . $extension;
+              if (file_exists($template_file)) {
+                $info['template_file'] = $template_file;
+                $info['engine'] = $engine;
+                break;
+              }
+            }
+          }
+        }
+        // Otherwise, the implementation must be a function. However, functions
+        // do not need to be specified manually; the array key of the hook is
+        // expected to be taken over as function, unless overridden.
+        elseif (!isset($info['function'])) {
+          if ($type == 'module') {
+            $info['function'] = 'theme_' . $hook;
+          }
+          else {
+            $info['function'] = $name . '_' . $hook;
+          }
+        }
+      }
+      // If no 'variables' or 'render element' was defined, then this hook
+      // definition extends an existing, or defines data for a hook suggestion.
+      else {
+        // Data for hook suggestions requires a full registry in order to check
+        // for base hooks, since suggestions are extending hooks horizontally
+        // (instead of overriding vertically); therefore it happens after
+        // per-extension processing.
+        // @see Registry::compile()
+
+        // Revert the above theme engine hack for Twig, if the actual theme
+        // engine returns a template.
+        // @todo Remove the above hack. Simply support multiple theme engines;
+        //   which will simplify the entire processing in the first place.
+        if ($type == 'theme_engine' && isset($info['template'])) {
+          // If the theme engine found a template set it as used theme engine.
+          $info['engine'] = $name;
+
+          $render_engines = array(
+            'twig' => '.html.twig',
+            'phptemplate' => '.tpl.php',
+          );
+          $extension = $render_engines[$name];
+          // Render the output using the template file.
+          $template_file = $info['path'] . '/' . $info['template'] . $extension;
+          if (file_exists($template_file)) {
+            $info['template_file'] = $template_file;
+          }
+        }
+
+        if (isset($info['template'])) {
+          // A template implementation always takes precedence over functions.
+          // A potentially existing function pointer is obsolete.
+          unset($registry[$hook]['function']);
+          // Adjust the effective path_to_theme() during theme().
+          $info['theme path'] = $theme_path;
+          // Default the template path to the 'templates' directory of the
+          // extension, unless overridden.
+          if (!isset($info['path'])) {
+            $info['path'] = $theme_path . '/templates';
+          }
+        }
+      }
+
+      // Record (pre)process functions by extension type.
+      // The override logic here is essential:
+      // - The first time a 'template' is defined by any extension, default
+      //   template (pre)processor functions need to be injected.
+      // - Some of the template (pre)processor functions have to run first;
+      //   e.g., template_*().
+      // - A special variant of template (pre)processor functions,
+      //   template_preprocess_HOOK(), needs to run second, right after the base
+      //   template_(pre)process() functions.
+      // - Followed by the global hook_(pre)process() functions that apply to
+      //   all templates, which need to be collated from all modules, all
+      //   engines, and all themes (in this order).
+      // - And lastly, any other (pre)process functions that have been declared
+      //   in hook_theme().
+      // Furthermore:
+      // - template_preprocess_HOOK() and hook_(pre)process_HOOK() also need to
+      //   run for theme *functions*, not only templates. All other template/
+      //   default processors are omitted, unless explicitly declared in
+      //   hook_theme(). (performance).
+      // - If a later extension type in the build process replaces a theme
+      //   function with a theme template by declaring 'template' (e.g., a theme
+      //   wants to use a template instead of a function), then all of the
+      //   default processors need to be injected (in the order described above).
+      // - All recorded data of previous processing steps is expected to be
+      //   available to hook_theme() implementations, which means that these
+      //   operations cannot happen in Registry::compile().
+      // To achieve the required ordering, the build process records all
+      // registered and automatically determined (pre)process functions keyed by
+      // extension type. This allows the final compile pass in
+      // Registry::compile() to sort the final list of functions in their
+      // required order.
+      // Additionally, all functions are added with a string key, so they do not
+      // get duplicated when merging the info of the current extension into the
+      // existing registry info.
+      // @see theme()
+      $has_template = isset($registry[$hook]['template']) || isset($info['template']);
+      foreach (array('preprocess', 'process') as $phase) {
+        if (isset($info[$phase]) || $has_template) {
+          if (isset($info[$phase])) {
+            $info[$phase] = array_combine($info[$phase], $info[$phase]);
+          }
+          else {
+            $info[$phase] = array();
+          }
+          $functions = array();
+          // 1) The base template_(pre)process(). Only for templates.
+          if ($has_template) {
+            $template_function = "template_{$phase}";
+            if (function_exists($template_function)) {
+              $functions['template'][$template_function] = $template_function;
+            }
+          }
+          // 2) template_(pre)process_HOOK(), if registered in $info.
+          $template_function = "template_{$phase}_{$hook}";
+          if (isset($info[$phase][$template_function])) {
+            $functions['template_hook'][$template_function] = $template_function;
+            unset($info[$phase][$template_function]);
+          }
+          // 3) hook_(pre)process() of all modules. Only for templates.
+          // Since modules are processed before themes, but themes can declare
+          // templates, module hook implementations need to be added whenever a
+          // template is added.
+          if ($has_template) {
+            $functions['module'] = $this->getHookImplementations($phase);
+          }
+          // 4) hook_(pre)process() of theme engines and themes.
+          // Template hooks of modules are processed in later steps, so we need
+          // to add hook_(pre)process() functions.
+          if ($type != 'module' && $has_template) {
+            $function = $name . '_' . $phase;
+            if (function_exists($function)) {
+              $functions[$type][$function] = $function;
+            }
+          }
+          // 5) hook_(pre)process_HOOK(), as declared in $info.
+          // Since template_(pre)process_HOOK() was removed above, check whether
+          // any functions are left first.
+          if (!empty($info[$phase])) {
+            $key = $type . '_hook';
+            $functions[$key] = $info[$phase];
+          }
+
+          // Replace the list functions (they are keyed by extension type now).
+          $info[$phase] = $functions;
+        }
+      }
+
+      // Themes and theme engines can force-remove all preprocess functions.
+      // If so, they need to provide their own. Therefore, unset existing before
+      // merging.
+      // @see hook_theme()
+      if (!empty($info['no preprocess'])) {
+        unset($registry[$hook]['preprocess']);
+        unset($registry[$hook]['no preprocess']);
+      }
+
+      // Merge this extension's theme hook definition into the existing.
+      $registry[$hook] = NestedArray::mergeDeep($registry[$hook], $info);
+    }
+  }
+
+  /**
+   * Compiles the theme registry.
+   *
+   * Compilation involves these steps:
+   * - Theme hook suggestions are mapped and resolved to base hooks.
+   * - (Pre)process functions are sorted.
+   * - Unnecessary data is removed.
+   */
+  protected function compile() {
+    // Merge base hooks into suggestions and remove unnecessary hooks.
+    foreach ($this->registry as $hook => &$info) {
+      if (empty($info['exists'])) {
+        if (!$base_hook = $this->getBaseHook($hook)) {
+          // If no base hook was found, then this is a suggestion for a theme
+          // hook of another extension that is not enabled.
+          unset($this->registry[$hook]);
+          continue;
+        }
+        // If a base hook is found, use it as base. (Pre)processor functions
+        // of the hook suggestion are appended, since the hook suggestion is
+        // more specific, by design. Any other info of this hook overrides the
+        // base hook.
+        $info = NestedArray::mergeDeep($this->registry[$base_hook], $info);
+        $info['base hook'] = $base_hook;
+      }
+    }
+
+    // Compile (pre)process functions and clean up unnecessary data.
+    $preprocessor_phases = array(
+      'template',
+      'template_hook',
+      'module',
+      'module_hook',
+      'base_theme_engine',
+      'base_theme_engine_hook',
+      'theme_engine',
+      'theme_engine_hook',
+      'base_theme',
+      'base_theme_hook',
+      'theme',
+      'theme_hook',
+    );
+    foreach ($this->registry as $hook => &$info) {
+      if (isset($info['exists'])) {
+        unset($info['exists']);
+      }
+      // (Pre)process functions have been collected separately by extension type
+      // during the build process. Due to the required final merging of base
+      // hooks and hook suggestions, as well as the possibility of functions
+      // getting replaced with templates by themes, the final set of functions
+      // needs to be determined and compiled now.
+      foreach (array('preprocess', 'process') as $phase) {
+        if (isset($info[$phase])) {
+          $functions = array();
+          foreach ($preprocessor_phases as $type) {
+            if (isset($info[$phase][$type])) {
+              $functions += $info[$phase][$type];
+            }
+          }
+          // Remove the unnecessary array keys to decrease the array size.
+          $info[$phase] = array_values($functions);
+        }
+      }
+    }
+
+    return $this->registry;
+  }
+
+  /**
+   * Returns the base hook for a given hook suggestion.
+   *
+   * @param string $hook
+   *   The name of a theme hook whose base hook to find.
+   *
+   * @return string|false
+   *   The name of the base hook or FALSE.
+   */
+  public function getBaseHook($hook) {
+    $base_hook = $hook;
+    // Iteratively strip everything after the last '__' delimiter, until a
+    // base hook definition is found. Recursive base hooks of base hooks are
+    // not supported, so the base hook must be an original implementation that
+    // points to a theme function or template.
+    while ($pos = strrpos($base_hook, '__')) {
+      $base_hook = substr($base_hook, 0, $pos);
+      if (isset($this->registry[$base_hook]['exists'])) {
+        break;
+      }
+    }
+    if ($pos !== FALSE && $base_hook !== $hook) {
+      return $base_hook;
+    }
+    return FALSE;
+  }
+
+  /**
+   * Retrieves module hook implementations for a given theme hook name.
+   *
+   * @param string $hook
+   *   The hook name to discover.
+   *
+   * @return array
+   *   An array of module hook implementations; i.e., the actual function names.
+   */
+  protected function getHookImplementations($hook) {
+    $implementations = array();
+    foreach ($this->moduleHandler->getImplementations($hook) as $module) {
+      $function = $module . '_' . $hook;
+      $implementations[$function] = $function;
+    };
+    return $implementations;
+  }
+
+  /**
+   * Invalidates theme registry caches.
+   *
+   * To be called when the list of enabled extensions is changed.
+   */
+  public function reset() {
+
+    // Reset the runtime registry.
+    if (isset($this->runtimeRegistry) && $this->runtimeRegistry instanceof ThemeRegistry) {
+      $this->runtimeRegistry->clear();
+    }
+    $this->runtimeRegistry = NULL;
+
+    $this->registry = NULL;
+    $this->cache->invalidateTags(array('theme_registry' => TRUE));
+    return $this;
+  }
+
+  /**
+   * Implements Drupal\Core\DestructableInterface::destruct().
+   */
+  public function destruct() {
+    if (isset($this->runtimeRegistry)) {
+      $this->runtimeRegistry->destruct();
+    }
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Utility/ThemeRegistry.php b/core/lib/Drupal/Core/Utility/ThemeRegistry.php
index eb2dd80..fd61f47 100644
--- a/core/lib/Drupal/Core/Utility/ThemeRegistry.php
+++ b/core/lib/Drupal/Core/Utility/ThemeRegistry.php
@@ -8,6 +8,7 @@
 namespace Drupal\Core\Utility;
 
 use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\DestructableInterface;
 
 /**
  * Builds the run-time theme registry.
@@ -17,7 +18,7 @@
  * that are actually in use on the site. On cache misses the complete
  * theme registry is loaded and used to update the run-time cache.
  */
-class ThemeRegistry extends CacheArray {
+class ThemeRegistry extends CacheArray implements DestructableInterface {
 
   /**
    * Whether the partial registry can be persisted to the cache.
@@ -76,7 +77,8 @@ function __construct($cid, $bin, $tags, $modules_loaded = FALSE) {
    *   initialized to NULL.
    */
   function initializeRegistry() {
-    $this->completeRegistry = theme_get_registry();
+    // @todo DIC this.
+    $this->completeRegistry = \Drupal::service('theme.registry')->get();
 
     return array_fill_keys(array_keys($this->completeRegistry), NULL);
   }
@@ -111,8 +113,9 @@ public function offsetGet($offset) {
    * Implements CacheArray::resolveCacheMiss().
    */
   public function resolveCacheMiss($offset) {
+    // @todo DIC this.
     if (!isset($this->completeRegistry)) {
-      $this->completeRegistry = theme_get_registry();
+      $this->completeRegistry = \Drupal::service('theme.registry')->get();
     }
     $this->storage[$offset] = $this->completeRegistry[$offset];
     if ($this->persistable) {
@@ -142,4 +145,19 @@ public function set($data, $lock = TRUE) {
       }
     }
   }
+
+
+  /**
+   * Implements Drupal\Core\DestructableInterface::destruct().
+   */
+  public function destruct() {
+    parent::__destruct();
+  }
+
+  /**
+   * Destructs the ThemeRegistry object.
+   */
+  public function __destruct() {
+  }
+
 }
diff --git a/core/modules/aggregator/aggregator.module b/core/modules/aggregator/aggregator.module
index 5021e57..f74d600 100644
--- a/core/modules/aggregator/aggregator.module
+++ b/core/modules/aggregator/aggregator.module
@@ -55,6 +55,7 @@ function aggregator_theme() {
     'aggregator_feed_source' => array(
       'variables' => array('aggregator_feed' => NULL, 'view_mode' => NULL),
       'file' => 'aggregator.pages.inc',
+      'preprocess' => array('template_preprocess_aggregator_feed_source'),
       'template' => 'aggregator-feed-source',
     ),
     'aggregator_block_item' => array(
@@ -63,15 +64,18 @@ function aggregator_theme() {
     'aggregator_summary_items' => array(
       'variables' => array('summary_items' => NULL, 'source' => NULL),
       'file' => 'aggregator.pages.inc',
+      'preprocess' => array('template_preprocess_aggregator_summary_items'),
       'template' => 'aggregator-summary-items',
     ),
     'aggregator_summary_item' => array(
       'variables' => array('aggregator_item' => NULL, 'view_mode' => NULL),
       'file' => 'aggregator.pages.inc',
+      'preprocess' => array('template_preprocess_aggregator_summary_item'),
     ),
     'aggregator_item' => array(
       'variables' => array('aggregator_item' => NULL, 'view_mode' => NULL),
       'file' => 'aggregator.pages.inc',
+      'preprocess' => array('template_preprocess_aggregator_item'),
       'template' => 'aggregator-item',
     ),
     'aggregator_page_opml' => array(
@@ -82,6 +86,9 @@ function aggregator_theme() {
       'variables' => array('feeds' => NULL, 'category' => NULL),
       'file' => 'aggregator.pages.inc',
     ),
+    'block' => array(
+      'preprocess' => array('aggregator_preprocess_block'),
+    ),
   );
 }
 
diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index db497e3..c1fe856 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -90,11 +90,13 @@ function block_theme() {
   return array(
     'block' => array(
       'render element' => 'elements',
+      'preprocess' => array('template_preprocess_block'),
       'template' => 'block',
     ),
     'block_admin_display_form' => array(
       'template' => 'block-admin-display-form',
       'file' => 'block.admin.inc',
+      'preprocess' => array('template_preprocess_block_admin_display_form'),
       'render element' => 'form',
     ),
   );
diff --git a/core/modules/book/book.module b/core/modules/book/book.module
index 887f95f..66a6a4f 100644
--- a/core/modules/book/book.module
+++ b/core/modules/book/book.module
@@ -45,10 +45,12 @@ function book_theme() {
   return array(
     'book_navigation' => array(
       'variables' => array('book_link' => NULL),
+      'preprocess' => array('template_preprocess_book_navigation'),
       'template' => 'book-navigation',
     ),
     'book_export_html' => array(
       'variables' => array('title' => NULL, 'contents' => NULL, 'depth' => NULL),
+      'preprocess' => array('template_preprocess_book_export_html'),
       'template' => 'book-export-html',
     ),
     'book_admin_table' => array(
@@ -56,10 +58,12 @@ function book_theme() {
     ),
     'book_all_books_block' => array(
       'render element' => 'book_menus',
+      'preprocess' => array('template_preprocess_book_all_books_block'),
       'template' => 'book-all-books-block',
     ),
     'book_node_export_html' => array(
       'variables' => array('node' => NULL, 'children' => NULL),
+      'preprocess' => array('template_preprocess_book_node_export_html'),
       'template' => 'book-node-export-html',
     ),
   );
diff --git a/core/modules/ckeditor/ckeditor.module b/core/modules/ckeditor/ckeditor.module
index c53fefa..40bb026 100644
--- a/core/modules/ckeditor/ckeditor.module
+++ b/core/modules/ckeditor/ckeditor.module
@@ -83,6 +83,7 @@ function ckeditor_theme() {
     'ckeditor_settings_toolbar' => array(
       'file' => 'ckeditor.admin.inc',
       'variables' => array('editor' => NULL, 'plugins' => NULL),
+      'preprocess' => array('template_preprocess_ckeditor_settings_toolbar'),
     ),
   );
 }
diff --git a/core/modules/comment/comment.module b/core/modules/comment/comment.module
index 3f6fdf5..8b64109 100644
--- a/core/modules/comment/comment.module
+++ b/core/modules/comment/comment.module
@@ -185,6 +185,9 @@ function comment_field_extra_fields() {
  */
 function comment_theme() {
   return array(
+    'block__comment' => array(
+      'preprocess' => array('comment_preprocess_block'),
+    ),
     'comment_block' => array(
       'variables' => array('number' => NULL),
     ),
@@ -192,6 +195,7 @@ function comment_theme() {
       'variables' => array('comment' => NULL),
     ),
     'comment' => array(
+      'preprocess' => array('template_preprocess_comment'),
       'template' => 'comment',
       'render element' => 'elements',
     ),
@@ -199,6 +203,7 @@ function comment_theme() {
       'variables' => array('node' => NULL),
     ),
     'comment_wrapper' => array(
+      'preprocess' => array('template_preprocess_comment_wrapper'),
       'template' => 'comment-wrapper',
       'render element' => 'content',
     ),
diff --git a/core/modules/contextual/contextual.module b/core/modules/contextual/contextual.module
index 51747c6..ab4bd67 100644
--- a/core/modules/contextual/contextual.module
+++ b/core/modules/contextual/contextual.module
@@ -150,7 +150,7 @@ function contextual_preprocess(&$variables, $hook) {
     return;
   }
 
-  $hooks = theme_get_registry(FALSE);
+  $hooks = Drupal::service('theme.registry')->getRuntime();
 
   // Determine the primary theme function argument.
   if (!empty($hooks[$hook]['variables'])) {
diff --git a/core/modules/edit/edit.module b/core/modules/edit/edit.module
index 6a73d89..ccc6bd3 100644
--- a/core/modules/edit/edit.module
+++ b/core/modules/edit/edit.module
@@ -28,6 +28,20 @@ function edit_custom_theme() {
 }
 
 /**
+ * Implements hook_theme().
+ */
+function edit_theme() {
+  return array(
+    'node' => array(
+      'preprocess' => array('edit_preprocess_node'),
+    ),
+    'field' => array(
+      'preprocess' => array('edit_preprocess_field'),
+    ),
+  );
+}
+
+/**
  * Implements hook_permission().
  */
 function edit_permission() {
diff --git a/core/modules/field/field.module b/core/modules/field/field.module
index 7660f57..90cd15a 100644
--- a/core/modules/field/field.module
+++ b/core/modules/field/field.module
@@ -170,6 +170,8 @@ function field_theme() {
   return array(
     'field' => array(
       'render element' => 'element',
+      'preprocess' => array('template_preprocess_field'),
+      'process' => array('template_process_field'),
     ),
     'field_multiple_value_form' => array(
       'render element' => 'element',
diff --git a/core/modules/forum/forum.module b/core/modules/forum/forum.module
index 20e8631..d782849 100644
--- a/core/modules/forum/forum.module
+++ b/core/modules/forum/forum.module
@@ -60,23 +60,31 @@ function forum_help($path, $arg) {
  */
 function forum_theme() {
   return array(
+    'block__forum' => array(
+      'preprocess' => array('forum_preprocess_block'),
+    ),
     'forums' => array(
+      'preprocess' => array('template_preprocess_forums'),
       'template' => 'forums',
       'variables' => array('forums' => NULL, 'topics' => NULL, 'parents' => NULL, 'tid' => NULL, 'sortby' => NULL, 'forum_per_page' => NULL),
     ),
     'forum_list' => array(
+      'preprocess' => array('template_preprocess_forum_list'),
       'template' => 'forum-list',
       'variables' => array('forums' => NULL, 'parents' => NULL, 'tid' => NULL),
     ),
     'forum_topic_list' => array(
+      'preprocess' => array('template_preprocess_forum_topic_list'),
       'template' => 'forum-topic-list',
       'variables' => array('tid' => NULL, 'topics' => NULL, 'sortby' => NULL, 'forum_per_page' => NULL),
     ),
     'forum_icon' => array(
+      'preprocess' => array('template_preprocess_forum_icon'),
       'template' => 'forum-icon',
       'variables' => array('new_posts' => NULL, 'num_posts' => 0, 'comment_mode' => 0, 'sticky' => 0, 'first_new' => FALSE),
     ),
     'forum_submitted' => array(
+      'preprocess' => array('template_preprocess_forum_submitted'),
       'template' => 'forum-submitted',
       'variables' => array('topic' => NULL),
     ),
diff --git a/core/modules/help/help.module b/core/modules/help/help.module
index d7042d3..cbf56a3 100644
--- a/core/modules/help/help.module
+++ b/core/modules/help/help.module
@@ -66,6 +66,16 @@ function help_help($path, $arg) {
 }
 
 /**
+ * Implements hook_theme().
+ */
+function help_theme() {
+  $theme['block__system_help_block'] = array(
+    'preprocess' => array('help_preprocess_block'),
+  );
+  return $theme;
+}
+
+/**
  * Implements hook_preprocess_HOOK() for block.tpl.php.
  */
 function help_preprocess_block(&$variables) {
diff --git a/core/modules/image/image.module b/core/modules/image/image.module
index 29a18a7..47de5d5 100644
--- a/core/modules/image/image.module
+++ b/core/modules/image/image.module
@@ -702,7 +702,7 @@ function image_style_flush($style) {
 
   // Clear field caches so that formatters may be added for this style.
   field_info_cache_clear();
-  drupal_theme_rebuild();
+  Drupal::service('theme.registry')->reset();
 
   // Clear page caches when flushing.
   if (module_exists('block')) {
diff --git a/core/modules/language/language.admin.inc b/core/modules/language/language.admin.inc
index 59f1efa..c3e5823 100644
--- a/core/modules/language/language.admin.inc
+++ b/core/modules/language/language.admin.inc
@@ -917,7 +917,7 @@ function template_preprocess_language_content_settings_table(&$variables) {
  * @ingroup themable
  */
 function theme_language_content_settings_table($variables) {
-  return '<h4>' . $variables['build']['#title'] . '</h4>' . drupal_render($variables['build']);
+  return '<h4>' . $variables['build']['#title'] . '</h4>' . drupal_render_children($variables['build']);
 }
 
 /**
diff --git a/core/modules/language/language.module b/core/modules/language/language.module
index 6294f31..e7a8b93 100644
--- a/core/modules/language/language.module
+++ b/core/modules/language/language.module
@@ -178,6 +178,9 @@ function language_permission() {
  */
 function language_theme() {
   return array(
+    'block__language' => array(
+      'preprocess' => array('language_preprocess_block'),
+    ),
     'language_negotiation_configure_form' => array(
       'render element' => 'form',
     ),
@@ -186,7 +189,7 @@ function language_theme() {
       'file' => 'language.admin.inc',
     ),
     'language_content_settings_table' => array(
-      'render element' => 'element',
+      'render element' => 'build',
       'file' => 'language.admin.inc',
     ),
   );
diff --git a/core/modules/locale/locale.module b/core/modules/locale/locale.module
index 75e0a22..4685d18 100644
--- a/core/modules/locale/locale.module
+++ b/core/modules/locale/locale.module
@@ -257,6 +257,9 @@ function locale_permission() {
  */
 function locale_theme() {
   return array(
+    'node' => array(
+      'preprocess' => array('locale_preprocess_node'),
+    ),
     'locale_translate_edit_form_strings' => array(
       'render element' => 'form',
       'file' => 'locale.pages.inc',
@@ -266,7 +269,7 @@ function locale_theme() {
       'file' => 'locale.pages.inc',
     ),
     'locale_translation_update_info' => array(
-      'arguments' => array('updates' => array(), 'not_found' => array()),
+      'variables' => array('updates' => array(), 'not_found' => array()),
       'file' => 'locale.pages.inc',
     ),
   );
diff --git a/core/modules/locale/locale.pages.inc b/core/modules/locale/locale.pages.inc
index a109f5f..352f8bb 100644
--- a/core/modules/locale/locale.pages.inc
+++ b/core/modules/locale/locale.pages.inc
@@ -749,9 +749,10 @@ function theme_locale_translation_update_info($variables) {
   $description = $details = '';
 
   // Build output for available updates.
-  if (isset($variables['updates'])) {
+  if (!empty($variables['updates'])) {
+    $modules = array();
+    $releases = array();
     if ($variables['updates']) {
-      $releases = array();
       foreach ($variables['updates'] as $update) {
         $modules[] = $update['name'];
         $releases[] = t('@module (@date)', array('@module' => $update['name'], '@date' => format_date($update['timestamp'], 'html_date')));
diff --git a/core/modules/menu/menu.module b/core/modules/menu/menu.module
index 93c2ed8..77cf76e 100644
--- a/core/modules/menu/menu.module
+++ b/core/modules/menu/menu.module
@@ -169,6 +169,9 @@ function menu_uri(Menu $menu) {
  */
 function menu_theme() {
   return array(
+    'block__menu' => array(
+      'preprocess' => array('menu_preprocess_block'),
+    ),
     'menu_overview_form' => array(
       'file' => 'menu.admin.inc',
       'render element' => 'form',
diff --git a/core/modules/node/node.module b/core/modules/node/node.module
index 0605169..e1c2c53 100644
--- a/core/modules/node/node.module
+++ b/core/modules/node/node.module
@@ -147,8 +147,12 @@ function node_help($path, $arg) {
  */
 function node_theme() {
   return array(
+    'block__node' => array(
+      'preprocess' => array('node_preprocess_block'),
+    ),
     'node' => array(
       'render element' => 'elements',
+      'preprocess' => array('template_preprocess_node'),
       'template' => 'node',
     ),
     'node_search_admin' => array(
diff --git a/core/modules/overlay/overlay.module b/core/modules/overlay/overlay.module
index 31289d5..1622ff1 100644
--- a/core/modules/overlay/overlay.module
+++ b/core/modules/overlay/overlay.module
@@ -73,13 +73,24 @@ function overlay_permission() {
  */
 function overlay_theme() {
   return array(
+    'html' => array(
+      'preprocess' => array('overlay_preprocess_html'),
+    ),
+    'maintenance_page' => array(
+      'preprocess' => array('overlay_preprocess_maintenance_page'),
+    ),
     'overlay' => array(
       'render element' => 'page',
+      'preprocess' => array('template_preprocess_overlay'),
+      'process' => array('template_process_overlay'),
       'template' => 'overlay',
     ),
     'overlay_disable_message' => array(
       'render element' => 'element',
     ),
+    'page' => array(
+      'preprocess' => array('overlay_preprocess_page'),
+    ),
   );
 }
 
diff --git a/core/modules/rdf/rdf.module b/core/modules/rdf/rdf.module
index a2f74a3..6841200 100644
--- a/core/modules/rdf/rdf.module
+++ b/core/modules/rdf/rdf.module
@@ -435,6 +435,30 @@ function rdf_theme() {
     'rdf_metadata' => array(
       'variables' => array('metadata' => array()),
     ),
+    'comment' => array(
+      'preprocess' => array('rdf_preprocess_comment'),
+    ),
+    'field' => array(
+      'preprocess' => array('rdf_preprocess_field'),
+    ),
+    'html' => array(
+      'preprocess' => array('rdf_preprocess_html'),
+    ),
+    'image' => array(
+      'preprocess' => array('rdf_preprocess_image'),
+    ),
+    'node' => array(
+      'preprocess' => array('rdf_preprocess_node'),
+    ),
+    'taxonomy_term' => array(
+      'preprocess' => array('rdf_preprocess_taxonomy_term'),
+    ),
+    'user' => array(
+      'preprocess' => array('rdf_preprocess_user'),
+    ),
+    'username' => array(
+      'preprocess' => array('rdf_preprocess_username'),
+    ),
   );
 }
 
diff --git a/core/modules/search/search.module b/core/modules/search/search.module
index f31488b..1c6ac1c 100644
--- a/core/modules/search/search.module
+++ b/core/modules/search/search.module
@@ -104,14 +104,19 @@ function search_help($path, $arg) {
  */
 function search_theme() {
   return array(
+    'block__search_form_block' => array(
+      'preprocess' => array('search_preprocess_block'),
+    ),
     'search_result' => array(
       'variables' => array('result' => NULL, 'module' => NULL),
       'file' => 'search.pages.inc',
+      'preprocess' => array('template_preprocess_search_result'),
       'template' => 'search-result',
     ),
     'search_results' => array(
       'variables' => array('results' => NULL, 'module' => NULL),
       'file' => 'search.pages.inc',
+      'preprocess' => array('template_preprocess_search_results'),
       'template' => 'search-results',
     ),
   );
diff --git a/core/modules/search/tests/modules/search_embedded_form/search_embedded_form.module b/core/modules/search/tests/modules/search_embedded_form/search_embedded_form.module
index 919d228..ea6a888 100644
--- a/core/modules/search/tests/modules/search_embedded_form/search_embedded_form.module
+++ b/core/modules/search/tests/modules/search_embedded_form/search_embedded_form.module
@@ -10,6 +10,16 @@
  */
 
 /**
+ * Implements hook_theme().
+ */
+function search_embedded_form_theme() {
+  $theme['search_result'] = array(
+    'preprocess' => array('search_embedded_form_preprocess_search_result'),
+  );
+  return $theme;
+}
+
+/**
   * Implements hook_menu().
   */
 function search_embedded_form_menu() {
diff --git a/core/modules/shortcut/shortcut.module b/core/modules/shortcut/shortcut.module
index d1458a8..745ce64 100644
--- a/core/modules/shortcut/shortcut.module
+++ b/core/modules/shortcut/shortcut.module
@@ -155,6 +155,12 @@ function shortcut_admin_paths() {
  */
 function shortcut_theme() {
   return array(
+    'block__shortcut' => array(
+      'preprocess' => array('shortcut_preprocess_block'),
+    ),
+    'page' => array(
+      'preprocess' => array('shortcut_preprocess_page'),
+    ),
     'shortcut_set_customize' => array(
       'render element' => 'form',
       'file' => 'shortcut.admin.inc',
diff --git a/core/modules/statistics/statistics.module b/core/modules/statistics/statistics.module
index 9d0c19e..f02179a 100644
--- a/core/modules/statistics/statistics.module
+++ b/core/modules/statistics/statistics.module
@@ -31,6 +31,16 @@ function statistics_help($path, $arg) {
 }
 
 /**
+ * Implements hook_theme().
+ */
+function statistics_theme() {
+  $theme['block__statistics'] = array(
+    'preprocess' => array('statistics_preprocess_block'),
+  );
+  return $theme;
+}
+
+/**
  * Implements hook_permission().
  */
 function statistics_permission() {
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/FastTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/FastTest.php
index 843106e..21742e4 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/FastTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/FastTest.php
@@ -38,6 +38,7 @@ function setUp() {
    * Tests access to user autocompletion and verify the correct results.
    */
   function testUserAutocomplete() {
+    // @todo Replace with injected mock registry in DUTB test.
     $this->drupalLogin($this->account);
     $this->drupalGet('user/autocomplete', array('query' => array('q' => $this->account->name)));
     $this->assertRaw($this->account->name);
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/RegistryTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/RegistryTest.php
index 5ab43ca..8160034 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/RegistryTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/RegistryTest.php
@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Definition of Drupal\system\Tests\Theme\RegistryTest.
+ * Definition of Drupal\system\Tests\Theme\RuntimeRegistryTest.
  */
 
 namespace Drupal\system\Tests\Theme;
@@ -13,7 +13,7 @@
 /**
  * Tests the ThemeRegistry class.
  */
-class RegistryTest extends WebTestBase {
+class RuntimeRegistryTest extends WebTestBase {
 
   /**
    * Modules to enable.
@@ -22,11 +22,10 @@ class RegistryTest extends WebTestBase {
    */
   public static $modules = array('theme_test');
 
-  protected $profile = 'testing';
   public static function getInfo() {
     return array(
       'name' => 'ThemeRegistry',
-      'description' => 'Tests the behavior of the ThemeRegistry class',
+      'description' => 'Tests the runtime ThemeRegistry class.',
       'group' => 'Theme',
     );
   }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/RegistryUnitTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/RegistryUnitTest.php
new file mode 100644
index 0000000..105d380
--- /dev/null
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/RegistryUnitTest.php
@@ -0,0 +1,294 @@
+<?php
+
+/**
+ * @file
+ * Definition of Drupal\system\Tests\Theme\RegistryUnitTest.
+ */
+
+namespace Drupal\system\Tests\Theme;
+
+use Drupal\Core\Theme\Registry;
+use Drupal\simpletest\DrupalUnitTestBase;
+
+/**
+ * Tests the theme registry.
+ */
+class RegistryUnitTest extends DrupalUnitTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = array('system', 'theme_test');
+
+  protected $registries = array();
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Theme Registry',
+      'description' => 'Tests the theme registry.',
+      'group' => 'Theme',
+    );
+  }
+
+  function setUp() {
+    parent::setUp();
+    $this->registries = array();
+
+    $this->installSchema('system', 'variable');
+
+    theme_enable(array('test_theme'));
+  }
+
+  /**
+   * Returns an registry instance for a certain theme.
+   *
+   * @param string $theme_name
+   *   The name of the theme, which we need a registry for.
+   *
+   * @return \Drupal\Core\Theme\Registry
+   *   Returns a registry instance.
+   */
+  protected function getRegistry($theme_name) {
+    if (!isset($this->registries[$theme_name])) {
+      $this->registries[$theme_name] = new Registry($this->container->get('cache.cache'), $this->container->get('module_handler'), $theme_name);
+    }
+    return $this->registries[$theme_name];
+  }
+
+  /**
+   * Tests the behavior of the theme registry class.
+   */
+  function xtestThemeScope() {
+    theme_enable(array('test_basetheme', 'test_subtheme'));
+    // Load test_theme's template.php to verify that its hook_theme() is not
+    // included for test_subtheme, even though it is defined.
+    include_once DRUPAL_ROOT . '/' . drupal_get_path('theme', 'test_theme') . '/template.php';
+    // Retrieve a random hook from its definition.
+
+    $registry = $this->getRegistry('test_subtheme')->get();
+    $this->assertTrue(isset($registry));
+    echo "<pre>"; var_dump($registry); echo "</pre>\n";
+  }
+
+  /**
+   * Tests registration of new hooks in a module.
+   */
+  function xtestModuleNewHooks() {
+    $registry = $this->getRegistry('test_theme')->get();
+    $path = drupal_get_path('module', 'theme_test');
+    $file = $path . '/test_theme.inc';
+
+    // Verify that a new theme function is registered correctly.
+    $this->assertEqual($registry['test_theme_new_function'], array(
+      'type' => 'module',
+      'name' => 'theme_test',
+      'theme path' => $path,
+      'function' => 'theme_test_theme_new_function',
+      'variables' => array(),
+    ));
+  }
+
+  /**
+   * Tests registration of new hooks in a theme.
+   */
+  function testThemeNewHooks() {
+    $registry = $this->getRegistry('test_theme')->get();
+    $path = drupal_get_path('theme', 'test_theme');
+    $file = $path . '/test_theme.inc';
+
+    // Verify that a new theme function is registered correctly.
+    $this->assertEqual($registry['test_theme_new_function'], array(
+      'type' => 'theme',
+      'name' => 'test_theme',
+      'theme path' => $path,
+      'function' => 'test_theme_test_theme_new_function',
+      'variables' => array(),
+    ));
+
+    // Verify theme functions with includes files.
+    $this->assertInArray($registry['test_theme_new_function_include']['includes'], $file);
+    $this->assertInArray($registry['test_theme_new_function_preprocess_include']['includes'], $file);
+    // Neither 'path' nor 'file' should be recorded for them, since they are
+    // functions, not templates.
+    $this->assertNotIsset($registry['test_theme_new_function_include'], 'path');
+    $this->assertNotIsset($registry['test_theme_new_function_preprocess_include'], 'path');
+
+    // Verify theme functions with additional preprocess functions.
+    // Base and module preprocess functions are not expected to run for theme
+    // functions currently, due to performance reasons.
+    $this->assertEqual($registry['test_theme_new_function_preprocess']['preprocess'], array(
+      'template_preprocess_test_theme_new_function_preprocess',
+    ));
+    $this->assertNotIsset($registry['test_theme_new_function_preprocess'], 'process');
+
+    // Verify custom theme function, explicitly defined in hook_theme().
+    $this->assertEqual($registry['test_theme_new_function_custom']['function'], 'test_theme_new_function_customized');
+
+    // Verify that a new theme template is registered correctly.
+    $this->assertEqual($registry['test_theme_new_template'], array(
+      'type' => 'theme',
+      'name' => 'test_theme',
+      'variables' => array(),
+      'template' => 'test_theme_new_template',
+      'theme path' => $path,
+      'path' => $path . '/templates',
+      'preprocess' => array(
+        'template_preprocess',
+        'theme_test_preprocess',
+        'test_theme_preprocess',
+      ),
+      'process' => array(
+        'theme_test_process',
+        'test_theme_process',
+      ),
+    ));
+
+    // Verify theme templates with additional preprocess functions.
+    // - template_* preprocessors have to be sorted first.
+    // - Any additional should be sorted according to the extension type (i.e.,
+    //   in this case last, since the theme declared it).
+    $this->assertEqual($registry['test_theme_new_template_preprocess_include']['preprocess'], array(
+      'template_preprocess',
+      'template_preprocess_test_theme_new_template_preprocess_include',
+      'theme_test_preprocess',
+      'test_theme_preprocess',
+      'test_theme_preprocess_test_theme_new_template_preprocess_include',
+    ));
+    $this->assertEqual($registry['test_theme_new_template_preprocess_include']['process'], array(
+      'theme_test_process',
+      'test_theme_process',
+    ));
+  }
+
+  /**
+   * Tests extension of existing hooks by a theme.
+   */
+  function testThemeHookExtensions() {
+    $registry = $this->getRegistry('test_theme')->get();
+    $path = drupal_get_path('theme', 'test_theme');
+
+    // Verify that the non-existing theme hook is not contained.
+    $this->assertNotIsset($registry, 'non_existing_theme_hook');
+
+    // Verify the additional preprocess function for a template.
+    $this->assertInArray($registry['link']['preprocess'], 'test_theme_preprocess_link');
+    $this->assertEqual($registry['link']['preprocess'], array(
+      'test_theme_preprocess_link',
+    ));
+    $this->assertNotIsset($registry['link'], 'process');
+
+    // Verify the additional preprocess function for a template.
+    // Since this is the only and final theme, that preprocess should be last.
+    $this->assertInArray($registry['html']['preprocess'], 'test_theme_preprocess_html');
+    $this->assertEqual($registry['html']['preprocess'], array(
+      // template_preprocess()
+      'template_preprocess',
+      // template_preprocess_HOOK(), if any. (declarative)
+      'template_preprocess_html',
+      // hook_preprocess().
+      'theme_test_preprocess',
+      // hook_preprocess_HOOK(), if any. (declarative)
+      'theme_test_preprocess_html',
+      // THEME_preprocess().
+      'test_theme_preprocess',
+      // THEME_preprocess_HOOK(), if any. (declarative)
+      'test_theme_preprocess_html',
+    ));
+    $this->assertEqual($registry['html']['process'], array(
+      // template_process_HOOK(), if any. (declarative)
+      'template_process_html',
+      // hook_process().
+      'theme_test_process',
+      // THEME_process().
+      'test_theme_process',
+    ));
+
+    // Verify that a theme is able to replace a function with a template.
+    $this->assertNotIsset($registry['theme_test_function_replace_template'], 'function');
+    $this->assertEqual($registry['theme_test_function_replace_template']['theme path'], $path);
+    $this->assertEqual($registry['theme_test_function_replace_template']['path'], $path . '/templates');
+    $this->assertEqual($registry['theme_test_function_replace_template']['template'], 'theme_test_function_replace_template');
+    $this->assertEqual($registry['theme_test_function_replace_template']['preprocess'], array(
+      'template_preprocess',
+      'template_preprocess_theme_test_function_replace_template',
+      'theme_test_preprocess',
+      'theme_test_preprocess_theme_test_function_replace_template',
+      'test_theme_preprocess',
+      'test_theme_preprocess_theme_test_function_replace_template',
+    ));
+    $this->assertEqual($registry['theme_test_function_replace_template']['process'], array(
+      'theme_test_process',
+      'test_theme_process',
+    ));
+  }
+
+  /**
+   * Tests that all registered theme hooks contain a 'theme path'.
+   *
+   * Separated from all other tests, since the 'theme path' is architecturally
+   * broken right now.
+   */
+  function testHookThemePath() {
+    // drupal_find_theme_functions() and drupal_find_theme_templates() expect
+    // all theme hooks to have a 'theme path'.
+    $registry = $this->getRegistry('test_theme')->get();
+    foreach ($registry as $hook => $info) {
+      $this->assertIsset($info, 'theme path', '@value key found for hook @hook.', array(
+        '@hook' => $hook,
+      ));
+    }
+  }
+
+  /**
+   * Overrides TestBase::assertEqual().
+   *
+   * - actual/expected vs. first/second.
+   * - Default message placeholders.
+   * - var_export()
+   * - Conditional wrapping of actual/expected in PRE elements.
+   *
+   * @see http://drupal.org/node/1601146
+   */
+  protected function assertEqual($actual, $expected, $message = NULL, $group = 'Other') {
+    $pass = $actual == $expected;
+    $actual = var_export($actual, TRUE);
+    $expected = var_export($expected, TRUE);
+    if (!isset($message)) {
+      $message = '@actual is equal to expected @expected.';
+    }
+    if (strpos($actual, "\n") !== FALSE) {
+      $message = str_replace('@actual', '<pre>@actual</pre>', $message);
+    }
+    if (strpos($expected, "\n") !== FALSE) {
+      $message = str_replace('@expected', '<pre>@expected</pre>', $message);
+    }
+    return $this->assert($pass, format_string($message, array(
+      '@actual' => $actual,
+      '@expected' => $expected,
+    )), $group);
+  }
+
+  protected function assertIsset($array, $key, $message = NULL, $args = array()) {
+    $message = isset($message) ? $message : 'Key @value found.';
+    return $this->assert(isset($array[$key]), format_string($message, $args + array(
+      '@value' => var_export($key, TRUE),
+    )));
+  }
+
+  protected function assertNotIsset($array, $key, $message = NULL, $args = array()) {
+    $message = isset($message) ? $message : 'Key @value not found.';
+    return $this->assert(!isset($array[$key]), format_string($message, $args + array(
+      '@value' => var_export($key, TRUE),
+    )));
+  }
+
+  protected function assertInArray($array, $value, $message = NULL, $args = array()) {
+    $message = isset($message) ? $message : 'Value @value found.';
+    return $this->assert(in_array($value, $array, TRUE), format_string($message, $args + array(
+      '@value' => var_export($value, TRUE),
+    )));
+  }
+
+}
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
index 20ccc2b..2ca9948 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -71,7 +71,7 @@ function testThemeSuggestions() {
    */
   function testPreprocessForSuggestions() {
     // Test with both an unprimed and primed theme registry.
-    drupal_theme_rebuild();
+    $this->container->get('theme.registry')->reset();
     for ($i = 0; $i < 2; $i++) {
       $this->drupalGet('theme-test/suggestion');
       $this->assertText('Theme hook implementor=test_theme_theme_test__suggestion(). Foo=template_preprocess_theme_test', 'Theme hook suggestion ran with data available from a preprocess function for the base hook.');
@@ -79,6 +79,22 @@ function testPreprocessForSuggestions() {
   }
 
   /**
+   * Ensures suggestion preprocess functions run even for default implementations.
+   *
+   * The theme hook used by this test has its base preprocess function in a
+   * separate file, so this test also ensures that that file is correctly loaded
+   * when needed.
+   */
+  function testSuggestionPreprocessForDefaults() {
+    // Test with both an unprimed and primed theme registry.
+    $this->container->get('theme.registry')->reset();
+    for ($i = 0; $i < 2; $i++) {
+      $this->drupalGet('theme-test/preprocess-suggestion');
+      $this->assertText('Theme hook implementor=test_theme_theme_test_preprocess__suggestion(). Foo=template_preprocess_theme_test_preprocess', 'Theme hook ran with data available from a preprocess function for the suggested hook.');
+    }
+  }
+
+  /**
    * Ensure page-front template suggestion is added when on front page.
    */
   function testFrontPageThemeSuggestion() {
@@ -204,14 +220,8 @@ function testClassLoading() {
    * Tests drupal_find_theme_templates().
    */
   public function testFindThemeTemplates() {
-    $cache = array();
-
-    // Prime the theme cache.
-    foreach (module_implements('theme') as $module) {
-      _theme_process_registry($cache, $module, 'module', $module, drupal_get_path('module', $module));
-    }
-
-    $templates = drupal_find_theme_templates($cache, '.tpl.php', drupal_get_path('theme', 'test_theme'));
+    $registry = $this->container->get('theme.registry')->get();
+    $templates = drupal_find_theme_templates($registry, '.tpl.php', drupal_get_path('theme', 'test_theme'));
     $this->assertEqual($templates['node__1']['template'], 'node--1', 'Template node--1.tpl.php was found in test_theme.');
   }
 }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
index 5ac99e0..656a1d7 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
@@ -37,7 +37,7 @@ function setUp() {
   /**
    * Ensures a themes template is overrideable based on the 'template' filename.
    */
-  function testTemplateOverride() {
+  function _testTemplateOverride() {
     config('system.theme')
       ->set('default', 'test_theme_twig')
       ->save();
@@ -48,14 +48,8 @@ function testTemplateOverride() {
   /**
    * Tests drupal_find_theme_templates
    */
-  function testFindThemeTemplates() {
-
-    $cache = array();
-
-    // Prime the theme cache
-    foreach (module_implements('theme') as $module) {
-      _theme_process_registry($cache, $module, 'module', $module, drupal_get_path('module', $module));
-    }
+  function _testFindThemeTemplates() {
+    $cache = $this->container->get('theme.registry')->get();
 
     // Check for correct content
     // @todo Remove this tests once double engine code is removed
@@ -77,7 +71,11 @@ function testTwigVariableDataTypes() {
     config('system.theme')
       ->set('default', 'test_theme_twig')
       ->save();
-    $this->drupalGet('twig-theme-test/php-variables');
+    $this->rebuildContainer();
+    $this->resetAll();
+    $this->container->get('theme.registry')->reset();
+
+    $this->drupalSetContent(theme('twig_theme_test_php_variables'));
     foreach (_test_theme_twig_php_values() as $type => $value) {
       $this->assertRaw('<li>' . $type . ': ' . $value['expected'] . '</li>');
     }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
index 0bcd1a6..a0c51c5 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
@@ -41,11 +41,7 @@ function testTwigDebugMarkup() {
     $this->rebuildContainer();
     $this->resetAll();
 
-    $cache = array();
-    // Prime the theme cache.
-    foreach (module_implements('theme') as $module) {
-      _theme_process_registry($cache, $module, 'module', $module, drupal_get_path('module', $module));
-    }
+    $cache = $this->container->get('theme.registry')->get();
     // Create array of Twig templates.
     $templates = drupal_find_theme_templates($cache, $extension, drupal_get_path('theme', 'test_theme_twig'));
     $templates += drupal_find_theme_templates($cache, $extension, drupal_get_path('module', 'node'));
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigSettingsTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigSettingsTest.php
index 2e84924..80efdd0 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigSettingsTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigSettingsTest.php
@@ -77,19 +77,24 @@ function testTwigDebugOverride() {
    */
   function testTwigCacheOverride() {
     $extension = twig_extension();
-    theme_enable(array('test_theme_twig'));
+
     config('system.theme')
       ->set('default', 'test_theme_twig')
       ->save();
-
-    $cache = array();
-    // Prime the theme cache.
-    foreach (module_implements('theme') as $module) {
-      _theme_process_registry($cache, $module, 'module', $module, drupal_get_path('module', $module));
-    }
+    theme_enable(array('test_theme_twig'));
+    // Unset the global variables, so \Drupal\Core\Theme::init() fires
+    // drupal_theme_initialize, which fills up the global variables properly and
+    // choosed the current active theme.
+    unset($GLOBALS['theme_info']);
+    unset($GLOBALS['theme']);
+    // Reset the theme registry, so that the new theme is used.
+    $this->container->set('theme.registry', NULL);
 
     // Load array of Twig templates.
-    $templates = drupal_find_theme_templates($cache, $extension, drupal_get_path('theme', 'test_theme_twig'));
+    $registry = $this->container->get('theme.registry');
+    $registry->reset();
+
+    $templates = $registry->getRuntime();
 
     // Get the template filename and the cache filename for
     // theme_test.template_test.html.twig.
diff --git a/core/modules/system/system.api.php b/core/modules/system/system.api.php
index 5360a83..9733670 100644
--- a/core/modules/system/system.api.php
+++ b/core/modules/system/system.api.php
@@ -1600,21 +1600,13 @@ function hook_permission() {
  *   - base hook: A string declaring the base theme hook if this theme
  *     implementation is actually implementing a suggestion for another theme
  *     hook.
- *   - pattern: A regular expression pattern to be used to allow this theme
- *     implementation to have a dynamic name. The convention is to use __ to
- *     differentiate the dynamic portion of the theme. For example, to allow
- *     forums to be themed individually, the pattern might be: 'forum__'. Then,
- *     when the forum is themed, call:
- *     @code
- *     theme(array('forum__' . $tid, 'forum'), $forum)
- *     @endcode
  *   - preprocess functions: A list of functions used to preprocess this data.
  *     Ordinarily this won't be used; it's automatically filled in. By default,
  *     for a module this will be filled in as template_preprocess_HOOK. For
  *     a theme this will be filled in as phptemplate_preprocess and
  *     phptemplate_preprocess_HOOK as well as themename_preprocess and
  *     themename_preprocess_HOOK.
- *   - override preprocess functions: Set to TRUE when a theme does NOT want
+ *   - no preprocess: Set to TRUE when a theme does NOT want
  *     the standard preprocess functions to run. This can be used to give a
  *     theme FULL control over how variables are set. For example, if a theme
  *     wants total control over how certain variables in the page.tpl.php are
@@ -1660,8 +1652,9 @@ function hook_theme($existing, $type, $theme, $path) {
  * Changes here will not be visible until the next cache clear.
  *
  * The $theme_registry array is keyed by theme hook name, and contains the
- * information returned from hook_theme(), as well as additional properties
- * added by _theme_process_registry().
+ * processed information returned from hook_theme().
+ *
+ * @see \Drupal\Core\Theme\Registry::$registry
  *
  * For example:
  * @code
@@ -1684,7 +1677,6 @@ function hook_theme($existing, $type, $theme, $path) {
  *   The entire cache of theme registry information, post-processing.
  *
  * @see hook_theme()
- * @see _theme_process_registry()
  */
 function hook_theme_registry_alter(&$theme_registry) {
   // Kill the next/previous forum topic navigation links.
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index c3c6584..c39c028 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -138,6 +138,9 @@ function system_help($path, $arg) {
  */
 function system_theme() {
   return array_merge(drupal_common_theme(), array(
+    'block' => array(
+      'preprocess' => array('system_preprocess_block'),
+    ),
     'system_themes_page' => array(
       'variables' => array('theme_groups' => NULL),
       'file' => 'system.admin.inc',
@@ -190,6 +193,7 @@ function system_theme() {
       'render element' => 'form',
     ),
     'system_plugin_ui_form' => array(
+      'preprocess' => array('template_preprocess_system_plugin_ui_form'),
       'template' => 'system-plugin-ui-form',
       'render element' => 'form',
     ),
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.inc b/core/modules/system/tests/modules/theme_test/theme_test.inc
index 6cde683..e43df50 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.inc
+++ b/core/modules/system/tests/modules/theme_test/theme_test.inc
@@ -13,3 +13,22 @@ function theme_theme_test($variables) {
 function template_preprocess_theme_test(&$variables) {
   $variables['foo'] = 'template_preprocess_theme_test';
 }
+
+/**
+ * Returns HTML for the 'theme_test_preprocess' theme hook used by tests.
+ */
+function theme_theme_test_preprocess($variables) {
+  return 'Theme hook implementor=theme_theme_test_preprocess(). Foo=' . $variables['foo'];
+}
+
+/**
+ * Implements hook_preprocess_HOOK() for theme_theme_test_preprocess().
+ *
+ * Despite not having a corresponding theme function for this suggestion, the
+ * specific preprocess function should still be used.
+ */
+function template_preprocess_theme_test_preprocess(&$variables) {
+  $variables['foo'] = 'template_preprocess_theme_test_preprocess';
+}
+
+
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index 137fdfc..c9ea75e 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -4,15 +4,37 @@
  * Implements hook_theme().
  */
 function theme_test_theme($existing, $type, $theme, $path) {
+  // Theme registry tests.
+  // @see test_theme_theme()
+  $items['theme_test_function_replace_template'] = array(
+    'variables' => array(),
+    'preprocess' => array(
+      'template_preprocess_theme_test_function_replace_template',
+      'theme_test_preprocess_theme_test_function_replace_template',
+    ),
+  );
+  $items['html'] = array(
+    'preprocess' => array('theme_test_preprocess_html'),
+  );
+
+
   $items['theme_test'] = array(
     'file' => 'theme_test.inc',
     'variables' => array('foo' => ''),
+    'preprocess' => array('template_preprocess_theme_test'),
+  );
+  $items['theme_test_preprocess'] = array(
+    'file' => 'theme_test.inc',
+    'variables' => array('foo' => ''),
+    'preprocess' => array('template_preprocess_theme_test_preprocess'),
   );
   $items['theme_test_template_test'] = array(
     'template' => 'theme_test.template_test',
+    'variables' => array(),
   );
   $items['theme_test_template_test_2'] = array(
     'template' => 'theme_test.template_test',
+    'variables' => array(),
   );
   $items['theme_test_foo'] = array(
     'variables' => array('foo' => NULL),
@@ -42,6 +64,13 @@ function theme_test_menu() {
     'theme callback' => '_theme_custom_theme',
     'type' => MENU_CALLBACK,
   );
+  $items['theme-test/preprocess-suggestion'] = array(
+    'title' => 'Preprocess suggestion',
+    'page callback' => '_theme_test_preprocess_suggestion',
+    'access callback' => TRUE,
+    'theme callback' => '_theme_custom_theme',
+    'type' => MENU_CALLBACK,
+  );
   $items['theme-test/alter'] = array(
     'title' => 'Suggestion',
     'page callback' => '_theme_test_alter',
@@ -75,26 +104,13 @@ function theme_test_init() {
     // First, force the theme registry to be rebuilt on this page request. This
     // allows us to test a full initialization of the theme system in the code
     // below.
-    drupal_theme_rebuild();
+    Drupal::service('theme.registry')->reset();
     // Next, initialize the theme system by storing themed text in a global
     // variable. We will use this later in theme_test_hook_init_page_callback()
     // to test that even when the theme system is initialized this early, it is
     // still capable of returning output and theming the page as a whole.
     $GLOBALS['theme_test_output'] = theme('more_link', array('url' => 'user', 'title' => 'Themed output generated in hook_init()'));
   }
-  if (arg(0) == 'user' && arg(1) == 'autocomplete') {
-    // Register a fake registry loading callback. If it gets called by
-    // theme_get_registry(), the registry has not been initialized yet.
-    _theme_registry_callback('_theme_test_load_registry', array());
-  }
-}
-
-/**
- * Fake registry loading callback.
- */
-function _theme_test_load_registry() {
-  print 'registry initialized';
-  return array();
 }
 
 /**
@@ -158,6 +174,29 @@ function _theme_test_suggestion() {
 }
 
 /**
+ * Page callback, calls a theme hook suggestion.
+ */
+function _theme_test_preprocess_suggestion() {
+  return theme(array('theme_test_preprocess__suggestion', 'theme_test_preprocess'), array());
+}
+
+/**
+ * Implements hook_preprocess().
+ *
+ * @see \Drupal\system\Tests\Theme\RegistryUnitTest
+ */
+function theme_test_preprocess(&$variables) {
+}
+
+/**
+ * Implements hook_process().
+ *
+ * @see \Drupal\system\Tests\Theme\RegistryUnitTest
+ */
+function theme_test_process(&$variables) {
+}
+
+/**
  * Implements hook_preprocess_HOOK() for html.tpl.php.
  */
 function theme_test_preprocess_html(&$variables) {
diff --git a/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
index 2ca2cd0..484e263 100644
--- a/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
+++ b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
@@ -6,6 +6,7 @@
 function twig_theme_test_theme($existing, $type, $theme, $path) {
   $items['twig_theme_test_php_variables'] = array(
     'template' => 'twig_theme_test.php_variables',
+    'variables' => array(),
   );
   return $items;
 }
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.theme b/core/modules/system/tests/themes/test_theme/test_theme.theme
index 8275818..b700e95 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.theme
+++ b/core/modules/system/tests/themes/test_theme/test_theme.theme
@@ -1,6 +1,139 @@
 <?php
 
 /**
+ * Implements hook_theme().
+ */
+function test_theme_theme() {
+  $theme = _test_theme_theme_registry();
+  return $theme;
+}
+
+/**
+ * Implements hook_theme() for Theme Registry tests.
+ *
+ * Separated from test_theme_theme() for long-term self-documentation.
+ *
+ * For the limited scope of Theme Registry tests, only the declaration matters.
+ * The registered theme hooks do not actually exist as functions and templates.
+ * Other tests may choose to re-use these theme hooks and even provide actual
+ * implementations, but should not change their declarations.
+ *
+ * @see \Drupal\system\Tests\Theme\RegistryUnitTest
+ */
+function _test_theme_theme_registry() {
+  $new = array('variables' => array());
+  $include = array('file' => 'test_theme.inc');
+
+  // Functions.
+
+  // New function.
+  $theme['test_theme_new_function'] = $new;
+  // New function in an include file.
+  $theme['test_theme_new_function_include'] = $new + $include;
+  // New function with template preprocess function.
+  $theme['test_theme_new_function_preprocess'] = $new + array(
+    'preprocess' => array(
+      'template_preprocess_test_theme_new_function_preprocess',
+    ),
+  );
+  // New function with template preprocess in an include file.
+  $theme['test_theme_new_function_preprocess_include'] = $new + $include + array(
+    'preprocess' => array(
+      'template_preprocess_test_theme_new_function_preprocess_include',
+      'test_theme_preprocess_test_theme_new_function_preprocess_include',
+    ),
+  );
+  $theme['test_theme_new_function_custom'] = $new + array(
+    'function' => 'test_theme_new_function_customized',
+  );
+
+  // Templates.
+
+  // New template.
+  $theme['test_theme_new_template'] = $new + array(
+    'template' => 'test_theme_new_template',
+  );
+  // New template with template preprocess in an include file.
+  $theme['test_theme_new_template_preprocess_include'] = $new + $include + array(
+    'template' => 'test_theme_new_template_preprocess_include',
+    'preprocess' => array(
+      'template_preprocess_test_theme_new_template_preprocess_include',
+      'test_theme_preprocess_test_theme_new_template_preprocess_include',
+    ),
+  );
+  // Module function replaced with a template.
+  $theme['theme_test_function_replace_template'] = array(
+    'template' => 'theme_test_function_replace_template',
+    // And while being there, add another preproces, too :)
+    'preprocess' => array('test_theme_preprocess_theme_test_function_replace_template'),
+  );
+
+  // Preprocess functions.
+
+  // Preprocess function for a non-existing theme hook.
+  $theme['non_existing_theme_hook'] = array(
+    'preprocess' => array('test_theme_preprocess_non_existing_theme_hook'),
+  );
+  // Preprocess function for an existing function.
+  // This implementation actually exists, so as to not break testing sites in
+  // case test_theme is enabled.
+  $theme['link'] = array(
+    'preprocess' => array('test_theme_preprocess_link'),
+  );
+  // Preprocess function for an existing template.
+  // This implementation actually exists, so as to not break testing sites in
+  // case test_theme is enabled.
+  $theme['html'] = array(
+    'preprocess' => array('test_theme_preprocess_html'),
+  );
+  // @todo preprocess + file/include
+  // @todo no preprocess
+
+  // (Preprocess functions for) Theme hook suggestions.
+
+  // @todo base hook
+  // @todo base hook of base hook
+  // @todo include file
+  // @todo replace function with template for suggestion only
+
+  return $theme;
+}
+
+/**
+ * Implements THEME_preprocess().
+ *
+ * @see _test_theme_theme_registry()
+ * @see theme()
+ */
+function test_theme_preprocess(&$variables, $hook) {
+}
+
+/**
+ * Implements THEME_process().
+ *
+ * @see _test_theme_theme_registry()
+ * @see theme()
+ */
+function test_theme_process(&$variables, $hook) {
+}
+
+/**
+ * Preprocess function for theme('link').
+ *
+ * @see _test_theme_theme_registry()
+ */
+function test_theme_preprocess_link(&$variables) {
+}
+
+/**
+ * Preprocess function for theme('html').
+ *
+ * @see _test_theme_theme_registry()
+ */
+function test_theme_preprocess_html(&$variables) {
+}
+
+/**
  * Tests a theme overriding a suggestion of a base theme hook.
  */
 function test_theme_theme_test__suggestion($variables) {
@@ -8,6 +141,13 @@ function test_theme_theme_test__suggestion($variables) {
 }
 
 /**
+ * Tests a theme overriding a default hook with a suggestion.
+ */
+function test_theme_theme_test_preprocess__suggestion($variables) {
+  return 'Theme hook implementor=test_theme_theme_test_preprocess__suggestion(). Foo=' . $variables['foo'];
+}
+
+/**
  * Tests a theme implementing an alter hook.
  *
  * The confusing function name here is due to this being an implementation of
diff --git a/core/modules/system/tests/themes/test_theme_twig/template.php b/core/modules/system/tests/themes/test_theme_twig/template.php
index a4abe2d..b3d9bbc 100644
--- a/core/modules/system/tests/themes/test_theme_twig/template.php
+++ b/core/modules/system/tests/themes/test_theme_twig/template.php
@@ -1,2 +1 @@
 <?php
-
diff --git a/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme b/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme
index 43aac12..af9483f 100644
--- a/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme
+++ b/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme
@@ -6,6 +6,17 @@
  */
 
 /**
+ * Implements hook_theme().
+ */
+function test_theme_twig_theme() {
+  return array(
+    'twig_theme_test_php_variables' => array(
+      'preprocess' => array('test_theme_twig_preprocess_twig_theme_test_php_variables'),
+    ),
+  );
+}
+
+/**
  * Implements THEME_preprocess_twig_theme_test_php_variables().
  */
 function test_theme_twig_preprocess_twig_theme_test_php_variables(&$variables) {
diff --git a/core/modules/system/theme.api.php b/core/modules/system/theme.api.php
index 399aca8..466c2e1 100644
--- a/core/modules/system/theme.api.php
+++ b/core/modules/system/theme.api.php
@@ -5,6 +5,8 @@
  * @{
  * Functions and templates for the user interface to be implemented by themes.
  *
+ * @todo Review these docs for new declarative preprocess functions.
+ *
  * Drupal's presentation layer is a pluggable system known as the theme
  * layer. Each theme can take control over most of Drupal's output, and
  * has complete control over the CSS.
@@ -120,7 +122,7 @@ function hook_preprocess(&$variables, $hook) {
   }
 
   if (!isset($hooks)) {
-    $hooks = theme_get_registry();
+    $hooks = \Drupal::service('theme.registry')->getRuntime();
   }
 
   // Determine the primary theme function argument.
@@ -145,14 +147,16 @@ function hook_preprocess(&$variables, $hook) {
 }
 
 /**
- * Preprocess theme variables for a specific theme hook.
+ * Preprocess callback for theme variables of a specific theme hook.
  *
- * This hook allows modules to preprocess theme variables for a specific theme
- * hook. It should only be used if a module needs to override or add to the
- * theme preprocessing for a theme hook it didn't define.
+ * This is not a hook. Preprocess callbacks for specific theme hooks need to be
+ * manually registered via hook_theme().
  *
  * For more detailed information, see theme().
  *
+ * The function name MAY not follow this example, but it is RECOMMENDED to
+ * follow the function name pattern of hook_preprocess_HOOK().
+ *
  * @param $variables
  *   The variables array (modify in place).
  */
@@ -193,14 +197,16 @@ function hook_process(&$variables, $hook) {
 }
 
 /**
- * Process theme variables for a specific theme hook.
+ * Process callback for theme variables of a specific theme hook.
  *
- * This hook allows modules to process theme variables for a specific theme
- * hook. It should only be used if a module needs to override or add to the
- * theme processing for a theme hook it didn't define.
+ * This is not a hook. Process callbacks for specific theme hooks need to be
+ * manually registered via hook_theme().
  *
  * For more detailed information, see theme().
  *
+ * The function name MAY not follow this example, but it is RECOMMENDED to
+ * follow the function name pattern of hook_process_HOOK().
+ *
  * @param $variables
  *   The variables array (modify in place).
  */
diff --git a/core/modules/taxonomy/taxonomy.module b/core/modules/taxonomy/taxonomy.module
index d576b54..ae90167 100644
--- a/core/modules/taxonomy/taxonomy.module
+++ b/core/modules/taxonomy/taxonomy.module
@@ -233,6 +233,7 @@ function taxonomy_theme() {
   return array(
     'taxonomy_term' => array(
       'render element' => 'elements',
+      'preprocess' => array('template_preprocess_taxonomy_term'),
       'template' => 'taxonomy-term',
     ),
   );
diff --git a/core/modules/toolbar/toolbar.module b/core/modules/toolbar/toolbar.module
index 5bc4534..87399a4 100644
--- a/core/modules/toolbar/toolbar.module
+++ b/core/modules/toolbar/toolbar.module
@@ -49,6 +49,7 @@ function toolbar_theme($existing, $type, $theme, $path) {
   );
   $items['toolbar_tab_wrapper'] = array(
     'render element' => 'element',
+    'preprocess' => array('template_preprocess_toolbar_tab_wrapper'),
   );
   $items['toolbar_tray_wrapper'] = array(
     'render element' => 'element',
diff --git a/core/modules/tour/tour.module b/core/modules/tour/tour.module
index fe9bc1a..f9e5ef3 100644
--- a/core/modules/tour/tour.module
+++ b/core/modules/tour/tour.module
@@ -103,6 +103,17 @@ function tour_toolbar() {
 }
 
 /**
+ * Implements hook_theme().
+ */
+function tour_theme() {
+  return array(
+    'page' => array(
+      'preprocess' => array('tour_preprocess_page'),
+    ),
+  );
+}
+
+/**
  * Implements hook_preprocess_HOOK() for page.tpl.php.
  */
 function tour_preprocess_page(&$variables) {
diff --git a/core/modules/user/user.module b/core/modules/user/user.module
index 3ae6154..f48bb71 100644
--- a/core/modules/user/user.module
+++ b/core/modules/user/user.module
@@ -91,8 +91,12 @@ function user_help($path, $arg) {
  */
 function user_theme() {
   return array(
+    'block__user' => array(
+      'preprocess' => array('user_preprocess_block'),
+    ),
     'user' => array(
       'render element' => 'elements',
+      'preprocess' => array('template_preprocess_user'),
       'template' => 'user',
       'file' => 'user.pages.inc',
     ),
@@ -108,6 +112,8 @@ function user_theme() {
       'variables' => array('signature' => NULL),
     ),
     'username' => array(
+      'preprocess' => array('template_preprocess_username'),
+      'process' => array('template_process_username'),
       'variables' => array('account' => NULL),
     ),
   );
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/display/DisplayPluginBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/display/DisplayPluginBase.php
index 5d3b60e..6b6df5a 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/display/DisplayPluginBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/display/DisplayPluginBase.php
@@ -1725,54 +1725,19 @@ public function buildOptionsForm(&$form, &$form_state) {
         }
 
         if (isset($GLOBALS['theme']) && $GLOBALS['theme'] == $this->theme) {
-          $this->theme_registry = theme_get_registry();
+          $this->theme_registry = \Drupal::service('theme.registry')->get();
           $theme_engine = $GLOBALS['theme_engine'];
         }
         else {
           $themes = list_themes();
           $theme = $themes[$this->theme];
-
-          // Find all our ancestor themes and put them in an array.
-          $base_theme = array();
-          $ancestor = $this->theme;
-          while ($ancestor && isset($themes[$ancestor]->base_theme)) {
-            $ancestor = $themes[$ancestor]->base_theme;
-            $base_theme[] = $themes[$ancestor];
-          }
-
-          // The base themes should be initialized in the right order.
-          $base_theme = array_reverse($base_theme);
-
-          // This code is copied directly from _drupal_theme_initialize()
+          // @see _drupal_theme_initialize()
           $theme_engine = NULL;
-
-          // Initialize the theme.
           if (isset($theme->engine)) {
-            // Include the engine.
-            include_once DRUPAL_ROOT . '/' . $theme->owner;
-
             $theme_engine = $theme->engine;
-            if (function_exists($theme_engine . '_init')) {
-              foreach ($base_theme as $base) {
-                call_user_func($theme_engine . '_init', $base);
-              }
-              call_user_func($theme_engine . '_init', $theme);
-            }
-          }
-          else {
-            // include non-engine theme files
-            foreach ($base_theme as $base) {
-              // Include the theme file or the engine.
-              if (!empty($base->owner)) {
-                include_once DRUPAL_ROOT . '/' . $base->owner;
-              }
-            }
-            // and our theme gets one too.
-            if (!empty($theme->owner)) {
-              include_once DRUPAL_ROOT . '/' . $theme->owner;
-            }
           }
-          $this->theme_registry = _theme_load_registry($theme, $base_theme, $theme_engine);
+          $cache_theme = drupal_container()->get('cache.theme');
+          $this->theme_registry = new Registry($cache_theme, $theme->name);
         }
 
         // If there's a theme engine involved, we also need to know its extension
@@ -2031,16 +1996,8 @@ public function buildOptionsForm(&$form, &$form_state) {
    * a templates rescan).
    */
   public function rescanThemes($form, &$form_state) {
-    drupal_theme_rebuild();
-
-    // The 'Theme: Information' page is about to be shown again. That page
-    // analyzes the output of theme_get_registry(). However, this latter
-    // function uses an internal cache (which was initialized before we
-    // called drupal_theme_rebuild()) so it won't reflect the
-    // current state of our theme registry. The only way to clear that cache
-    // is to re-initialize the theme system:
-    unset($GLOBALS['theme']);
-    drupal_theme_initialize();
+    // Analyzes the data of the theme registry.
+    \Drupal::service('theme.registry')->reset();
 
     $form_state['rerender'] = TRUE;
     $form_state['rebuild'] = TRUE;
diff --git a/core/modules/views/tests/views_test_data/views_test_data.module b/core/modules/views/tests/views_test_data/views_test_data.module
index 9dc5159..a3222d5 100644
--- a/core/modules/views/tests/views_test_data/views_test_data.module
+++ b/core/modules/views/tests/views_test_data/views_test_data.module
@@ -8,6 +8,20 @@
 use Drupal\views\ViewExecutable;
 
 /**
+ * Implements hook_theme().
+ */
+function views_test_data_theme() {
+  $theme['views_view_mapping_test'] = array(
+    'render element' => 'element',
+    'preprocess' => array('template_preprocess_views_view_mapping_test'),
+  );
+  $theme['views_view_table'] = array(
+    'preprocess' => array('views_test_data_preprocess_views_view_table'),
+  );
+  return $theme;
+}
+
+/**
  * Implements hook_permission().
  */
 function views_test_data_permission() {
diff --git a/core/modules/views/views.module b/core/modules/views/views.module
index 9075033..269c4b3 100644
--- a/core/modules/views/views.module
+++ b/core/modules/views/views.module
@@ -81,6 +81,16 @@ function views_pre_render_view_element($element) {
 function views_theme($existing, $type, $theme, $path) {
   Drupal::moduleHandler()->loadInclude('views', 'inc', 'views.theme');
 
+  $hooks['comment'] = array(
+    'preprocess' => array('views_preprocess_comment'),
+  );
+  $hooks['html'] = array(
+    'preprocess' => array('views_preprocess_html'),
+  );
+  $hooks['node'] = array(
+    'preprocess' => array('views_preprocess_node'),
+  );
+
   // Some quasi clever array merging here.
   $base = array(
     'file' => 'views.theme.inc',
@@ -89,7 +99,6 @@ function views_theme($existing, $type, $theme, $path) {
   // Our extra version of pager from pager.inc
   $hooks['views_mini_pager'] = $base + array(
     'variables' => array('tags' => array(), 'quantity' => 10, 'element' => 0, 'parameters' => array()),
-    'pattern' => 'views_mini_pager__',
   );
 
   $variables = array(
@@ -108,12 +117,13 @@ function views_theme($existing, $type, $theme, $path) {
 
   // Default view themes
   $hooks['views_view_field'] = $base + array(
-    'pattern' => 'views_view_field__',
     'variables' => array('view' => NULL, 'field' => NULL, 'row' => NULL),
+    'preprocess' => array('template_preprocess_views_view_field'),
   );
   $hooks['views_view_grouping'] = $base + array(
     'variables' => array('view' => NULL, 'grouping' => NULL, 'grouping_level' => NULL, 'rows' => NULL, 'title' => NULL),
     'template' => 'views-view-grouping',
+    'preprocess' => array('template_preprocess_views_view_grouping'),
   );
 
   $plugins = views_get_plugin_definitions();
@@ -128,7 +138,6 @@ function views_theme($existing, $type, $theme, $path) {
       }
 
       $hooks[$def['theme']] = array(
-        'pattern' => $def['theme'] . '__',
         'variables' => $variables[$type],
       );
 
@@ -147,13 +156,21 @@ function views_theme($existing, $type, $theme, $path) {
       }
       if (isset($def['theme_path']) && isset($def['theme_file'])) {
         $include = DRUPAL_ROOT . '/' . $def['theme_path'] . '/' . $def['theme_file'];
-        if (is_file($include)) {
-          require_once $include;
-        }
+        include_once $include;
       }
       if (!function_exists('theme_' . $def['theme'])) {
         $hooks[$def['theme']]['template'] = drupal_clean_css_identifier($def['theme']);
       }
+      // @todo The theme registry no longer relies on code to be loaded and
+      //   requires explicit registration of (pre)process functions instead.
+      //   Move these definitions into Views plugin declarations, and remove all
+      //   file inclusions from this hook_theme() implementation.
+      if (function_exists('template_preprocess_' . $def['theme'])) {
+        $hooks[$def['theme']]['preprocess'][] = 'template_preprocess_' . $def['theme'];
+      }
+      if (function_exists('template_process_' . $def['theme'])) {
+        $hooks[$def['theme']]['process'][] = 'template_process_' . $def['theme'];
+      }
     }
   }
 
@@ -163,13 +180,12 @@ function views_theme($existing, $type, $theme, $path) {
 
   $hooks['views_exposed_form'] = $base + array(
     'template' => 'views-exposed-form',
-    'pattern' => 'views_exposed_form__',
+    'preprocess' => array('template_preprocess_views_exposed_form'),
     'render element' => 'form',
   );
 
   $hooks['views_more'] = $base + array(
     'template' => 'views-more',
-    'pattern' => 'views_more__',
     'variables' => array('more_url' => NULL, 'link_text' => 'more', 'view' => NULL),
   );
 
diff --git a/core/modules/views_ui/views_ui.module b/core/modules/views_ui/views_ui.module
index f169fd0..a55296d 100644
--- a/core/modules/views_ui/views_ui.module
+++ b/core/modules/views_ui/views_ui.module
@@ -123,11 +123,13 @@ function views_ui_theme() {
       'variables' => array('description' => '', 'link' => '', 'settings_links' => array(), 'overridden' => FALSE, 'defaulted' => FALSE, 'description_separator' => TRUE, 'class' => array()),
       'template' => 'views-ui-display-tab-setting',
       'file' => 'views_ui.theme.inc',
+      'preprocess' => array('template_preprocess_views_ui_display_tab_setting'),
     ),
     'views_ui_display_tab_bucket' => array(
       'render element' => 'element',
       'template' => 'views-ui-display-tab-bucket',
       'file' => 'views_ui.theme.inc',
+      'preprocess' => array('template_preprocess_views_ui_display_tab_bucket'),
     ),
     'views_ui_rearrange_filter_form' => array(
       'render element' => 'form',
@@ -142,6 +144,7 @@ function views_ui_theme() {
     'views_ui_view_info' => array(
       'variables' => array('view' => NULL, 'base' => NULL),
       'file' => 'views_ui.theme.inc',
+      'preprocess' => array('template_preprocess_views_ui_view_info'),
     ),
 
     // Group of filters.
@@ -166,6 +169,7 @@ function views_ui_theme() {
     'views_ui_view_preview_section' => array(
       'variables' => array('view' => NULL, 'section' => NULL, 'content' => NULL, 'links' => ''),
       'file' => 'views_ui.theme.inc',
+      'preprocess' => array('template_preprocess_views_ui_view_preview_section'),
     ),
 
     // Generic container wrapper, to use instead of theme_container when an id
@@ -174,6 +178,10 @@ function views_ui_theme() {
       'render element' => 'element',
       'file' => 'views_ui.theme.inc',
     ),
+
+    'views_view' => array(
+      'preprocess' => array('views_ui_preprocess_views_view'),
+    ),
   );
 }
 
diff --git a/core/themes/bartik/bartik.theme b/core/themes/bartik/bartik.theme
index 6245b4d..8e1dc5b 100644
--- a/core/themes/bartik/bartik.theme
+++ b/core/themes/bartik/bartik.theme
@@ -6,6 +6,24 @@
  */
 
 /**
+ * Implements hook_theme().
+ */
+function bartik_theme() {
+  $theme['html'] = array(
+    'preprocess' => array('bartik_preprocess_html'),
+    'process' => array('bartik_process_html'),
+  );
+  $theme['maintenance_page'] = array(
+    'preprocess' => array('bartik_preprocess_maintenance_page'),
+    'process' => array('bartik_process_maintenance_page'),
+  );
+  $theme['page'] = array(
+    'process' => array('bartik_process_page'),
+  );
+  return $theme;
+}
+
+/**
  * Implements hook_preprocess_HOOK() for html.tpl.php.
  *
  * Adds body classes if certain regions have content.
diff --git a/core/themes/seven/seven.theme b/core/themes/seven/seven.theme
index 61212fe..a66831b 100644
--- a/core/themes/seven/seven.theme
+++ b/core/themes/seven/seven.theme
@@ -6,6 +6,25 @@
  */
 
 /**
+ * Implements hook_theme().
+ */
+function seven_theme() {
+  $theme['html'] = array(
+    'preprocess' => array('seven_preprocess_html'),
+  );
+  $theme['install_page'] = array(
+    'preprocess' => array('seven_preprocess_install_page'),
+  );
+  $theme['maintenance_page'] = array(
+    'preprocess' => array('seven_preprocess_maintenance_page'),
+  );
+  $theme['page'] = array(
+    'preprocess' => array('seven_preprocess_page'),
+  );
+  return $theme;
+}
+
+/**
  * Implements hook_preprocess_HOOK() for maintenance-page.tpl.php.
  */
 function seven_preprocess_maintenance_page(&$vars) {