diff --git includes/bootstrap.inc includes/bootstrap.inc
index 4cbfc3f..5a75dbe 100644
--- includes/bootstrap.inc
+++ includes/bootstrap.inc
@@ -773,6 +773,129 @@ function variable_initialize($conf = array()) {
 }
 
 /**
+ * Get the name of the currently active install profile.
+ *
+ * When this function is called during Drupal's initial installation process,
+ * the name of the profile that's about to be installed is stored in the global
+ * installation state. At all other times, the standard Drupal systems variable
+ * table contains the name of the current profile, and we can call variable_get()
+ * to determine what one is active.
+ *
+ * @return $profile
+ *   The name of the install profile.
+ */
+function drupal_get_profile() {
+  global $install_state;
+
+  if (isset($install_state['parameters']['profile'])) {
+    $profile = $install_state['parameters']['profile'];
+  }
+  else {
+    $profile = variable_get('install_profile', 'standard');
+  }
+
+  return $profile;
+}
+
+/**
+ * Add directories to search for system object files (modules, themes, etc.).
+ *
+ * This function is used to add directories to be searched when we build
+ * the system table. This function also manages the order of precedence
+ * of modules / themes found in these directories.
+ *
+ * There are three main levels of precedence, namely 'site' modules override
+ * 'profile' modules which override 'system' modules. You are also able to
+ * add a directory before or after any of these levels by prepending the
+ * scope variable with 'before:' or 'after:'.
+ *
+ * For instance, if you want to add a directory to search for additional modules
+ * that should override modules in your /sites/$sitename/ directory, you can
+ * use :
+ *    drupal_add_system_directory('after:site', 'custom/path');
+ *
+ * If no additional paths have been configured, the default directories are :
+ *
+ *   system - $directory
+ *   profile - profiles/$profilename/$directory
+ *   before:site - sites/all/$directory
+ *   site - sites/$sitename/$directory
+ *
+ * @param $scope
+ *   The scope of the include path to be added. May be one of the following:
+ *   - before:system
+ *   - system
+ *   - after:system
+ *   - before:profile
+ *   - profile
+ *   - after:profile
+ *   - before:site
+ *   - site
+ *   - after:site
+ *   - host_profile
+ *
+ * @param $directory
+ *   The relative path to be added.
+ *
+ *  @return
+ *    If $scope is null, return the list of directories.
+ */
+function drupal_add_system_directory($scope = null, $directory = null) {
+  static $search_dirs = array();
+
+  // Pre-load the directory array with the know directory scopes.
+  if (!sizeof($search_dirs)) {
+    // Drupal base system directories.
+    $search_dirs['before:system'] = array();
+    $search_dirs['system'] = array('.');
+    $search_dirs['after:system'] = array();
+
+    // The 'profiles' directory contains pristine collections of modules and
+    // themes as organized by a distribution. It is pristine in the same way
+    // that /modules is pristine for core; users should avoid changing anything
+    // there in favor of sites/all or sites/<domain> directories.
+    $search_dirs['before:profile'] = array();
+    $search_dirs['profile'] = array();
+    $search_dirs['after:profile'] = array();
+
+    // Always search sites/all/* as well as the global directories
+    $search_dirs['before:site'] = array('sites/all');
+    // The configuraton path.
+    $search_dirs['site'] = array();
+    $search_dirs['after:site'] = array();
+  }
+
+  // Return the entire list of directories.
+  if (is_null($scope) && is_null($directory)) {
+    return $search_dirs;
+  }
+
+  if (!is_null($directory) && !is_null($scope)) {
+    $search_dirs[$scope][] = $directory;
+  }
+
+  if (array_key_exists($scope, $search_dirs)) {
+    return $search_dirs[$scope];
+  }
+
+  return FALSE;
+}
+
+/**
+ * Returns directories searched for system object files (modules, themes etc.).
+ *
+ * This function is used to return the directories that have been added with
+ * drupal_add_system_directory().
+ *
+ * @return
+ *   Array containing directory paths.
+ */
+function drupal_get_system_directory() {
+  return drupal_add_system_directory();
+}
+
+
+/**
  * Returns a persistent variable.
  *
  * Case-sensitivity of the variable_* functions depends on the database
@@ -2089,6 +2212,9 @@ function _drupal_bootstrap_configuration() {
   timer_start('page');
   // Initialize the configuration, including variables from settings.php.
   drupal_settings_initialize();
+
+  // Add the config path to the search path.
+  drupal_add_system_directory('site', conf_path());
 }
 
 /**
@@ -2220,6 +2346,10 @@ function _drupal_bootstrap_variables() {
 
   // Load variables from the database, but do not overwrite variables set in settings.php.
   $conf = variable_initialize(isset($conf) ? $conf : array());
+
+  // Add the installation profile to the system directories.
+  drupal_add_system_directory('profile', "profiles/" . drupal_get_profile());
+
   // Load bootstrap modules.
   require_once DRUPAL_ROOT . '/includes/module.inc';
   module_load_all(TRUE);
diff --git includes/common.inc includes/common.inc
index 5a7e340..f03fb6b 100644
--- includes/common.inc
+++ includes/common.inc
@@ -195,32 +195,6 @@ function drupal_get_region_content($region = NULL, $delimiter = ' ') {
 }
 
 /**
- * Get the name of the currently active install profile.
- *
- * When this function is called during Drupal's initial installation process,
- * the name of the profile that's about to be installed is stored in the global
- * installation state. At all other times, the standard Drupal systems variable
- * table contains the name of the current profile, and we can call variable_get()
- * to determine what one is active.
- *
- * @return $profile
- *   The name of the install profile.
- */
-function drupal_get_profile() {
-  global $install_state;
-
-  if (isset($install_state['parameters']['profile'])) {
-    $profile = $install_state['parameters']['profile'];
-  }
-  else {
-    $profile = variable_get('install_profile', 'standard');
-  }
-
-  return $profile;
-}
-
-
-/**
  * Set the breadcrumb trail for the current page.
  *
  * @param $breadcrumb
@@ -4755,6 +4729,7 @@ function drupal_cron_cleanup() {
   }
 }
 
+
 /**
  * Returns information about system object files (modules, themes, etc.).
  *
@@ -4803,26 +4778,16 @@ function drupal_cron_cleanup() {
  *   - 'name': Name of file without the extension.
  */
 function drupal_system_listing($mask, $directory, $key = 'name', $min_depth = 1) {
-  $config = conf_path();
-
-  $profile = drupal_get_profile();
-
-  $searchdir = array($directory);
+  $searchdir = array();
   $files = array();
 
-  // The 'profiles' directory contains pristine collections of modules and
-  // themes as organized by a distribution. It is pristine in the same way
-  // that /modules is pristine for core; users should avoid changing anything
-  // there in favor of sites/all or sites/<domain> directories.
-  if (file_exists("profiles/$profile/$directory")) {
-    $searchdir[] = "profiles/$profile/$directory";
-  }
-
-  // Always search sites/all/* as well as the global directories
-  $searchdir[] = 'sites/all/' . $directory;
-
-  if (file_exists("$config/$directory")) {
-    $searchdir[] = "$config/$directory";
+  // Include directories from the drupal_add_system_directory() function.
+  foreach (drupal_get_system_directory() as $scope) {
+    foreach ($scope as $search) {
+      if (!is_null($search) && file_exists("$search/$directory") && !isset($searchdir[$search])) {
+        $searchdir[$search] = ($search == '.') ? $directory : "$search/$directory";
+      }
+    }
   }
 
   // Get current list of items
diff --git includes/install.core.inc includes/install.core.inc
index 3b39e08..6c5a5db 100644
--- includes/install.core.inc
+++ includes/install.core.inc
@@ -572,6 +572,8 @@ function install_tasks($install_state) {
 
   // Now add any tasks defined by the installation profile.
   if (!empty($install_state['parameters']['profile'])) {
+    drupal_add_system_directory('profile', 'profiles/' . $install_state['parameters']['profile']);
+
     $function = $install_state['parameters']['profile'] . '_install_tasks';
     if (function_exists($function)) {
       $result = $function($install_state);
diff --git modules/simpletest/drupal_web_test_case.php modules/simpletest/drupal_web_test_case.php
index 257921a..917cdd7 100644
--- modules/simpletest/drupal_web_test_case.php
+++ modules/simpletest/drupal_web_test_case.php
@@ -1192,6 +1192,11 @@ class DrupalWebTestCase extends DrupalTestCase {
     $this->originalProfile = drupal_get_profile();
     $clean_url_original = variable_get('clean_url', 0);
 
+    // The host profile's collection of modules or themes need to be included
+    // when running tests that use an alternate install profile but still reside
+    // in the host profile's directory.
+    drupal_add_system_directory('after:profile', 'profiles/' . $this->originalProfile);
+
     // Save and clean shutdown callbacks array because it static cached and
     // will be changed by the test run. If we don't, then it will contain
     // callbacks from both environments. So testing environment will try