diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemePrepareTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemePrepareTest.php
index 9614924..ecd4d56 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemePrepareTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemePrepareTest.php
@@ -29,31 +29,37 @@ public static function getInfo() {
     );
   }
 
-  function setUp() {
+  public function setUp() {
     parent::setUp();
-    theme_enable(array('test_theme'));
+    theme_enable(array('test_theme', 'test_basetheme', 'test_subtheme'));
   }
 
-  function testThemePrepare() {
+  /**
+   * Ensures prepare hooks work and are invoked and in the correct order.
+   */
+  public function testThemePrepare() {
     $this->drupalGet('theme-test/prepare');
 
     $this->assertRaw('<p>moduleVariable1: Drupal\theme_test\ThemeTestController::prepare<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleVariable2: Drupal\theme_test\ThemeTestController::prepare<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleVariableSuggestion1: Drupal\theme_test\ThemeTestController::prepare<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleVariableSuggestion2: Drupal\theme_test\ThemeTestController::prepare<br>
 theme_test_theme_prepare_theme_test_prepare__implemented<br>
@@ -62,7 +68,9 @@ function testThemePrepare() {
 test_theme_theme_prepare_theme_test_prepare__implemented<br>
 theme_test_theme_prepare_alter<br>
 test_theme_theme_prepare_alter<br>
-theme_test_theme_prepare_theme_test_prepare__implemented_alter</p>');
+test_theme_theme_prepare_theme_test_prepare_alter<br>
+theme_test_theme_prepare_theme_test_prepare__implemented_alter<br>
+test_theme_theme_prepare_theme_test_prepare__implemented_alter</p>');
 
     $this->assertRaw('<p>moduleVariableSuggestion3: Drupal\theme_test\ThemeTestController::prepare<br>
 theme_test_theme_prepare_theme_test_prepare__implemented__extended<br>
@@ -71,127 +79,179 @@ function testThemePrepare() {
 test_theme_theme_prepare_theme_test_prepare__implemented__extended<br>
 theme_test_theme_prepare_alter<br>
 test_theme_theme_prepare_alter<br>
-theme_test_theme_prepare_theme_test_prepare__implemented__extended_alter</p>');
+test_theme_theme_prepare_theme_test_prepare_alter<br>
+theme_test_theme_prepare_theme_test_prepare__implemented__extended_alter<br>
+test_theme_theme_prepare_theme_test_prepare__implemented__extended_alter</p>');
 
     $this->assertRaw('<p>moduleElement1: Drupal\theme_test\ThemeTestController::prepare<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleInjectedVariable1: theme_test_theme_prepare<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleInjectedVariable2: theme_test_theme_prepare_theme_test_prepare_element<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>themeInjectedVariable1: test_theme_theme_prepare<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleInjectedVariable3: theme_test_theme_prepare_theme_test_prepare_element_alter<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleElementSuggestion1: Drupal\theme_test\ThemeTestController::prepare<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleInjectedVariable1: theme_test_theme_prepare<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleInjectedVariable2: theme_test_theme_prepare_theme_test_prepare_element<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>themeInjectedVariable1: test_theme_theme_prepare<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleInjectedVariable3: theme_test_theme_prepare_theme_test_prepare_element_alter<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleElementSuggestion2: Drupal\theme_test\ThemeTestController::prepare<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleInjectedVariable1: theme_test_theme_prepare<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleInjectedVariable2: theme_test_theme_prepare_theme_test_prepare_element<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>themeInjectedVariable1: test_theme_theme_prepare<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleInjectedVariable3: theme_test_theme_prepare_theme_test_prepare_element_alter<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleElementSuggestion3: Drupal\theme_test\ThemeTestController::prepare<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleInjectedVariable1: theme_test_theme_prepare<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleInjectedVariable2: theme_test_theme_prepare_theme_test_prepare_element<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>themeInjectedVariable1: test_theme_theme_prepare<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
 
     $this->assertRaw('<p>moduleInjectedVariable3: theme_test_theme_prepare_theme_test_prepare_element_alter<br>
 test_theme_theme_prepare<br>
 test_theme_theme_prepare_theme_test_prepare<br>
 theme_test_theme_prepare_alter<br>
-test_theme_theme_prepare_alter</p>');
+test_theme_theme_prepare_alter<br>
+test_theme_theme_prepare_theme_test_prepare_alter</p>');
+  }
+
+  /**
+   * Ensures prepare hooks for subthemes work.
+   */
+  function testThemePrepareSubtheme() {
+    \Drupal::config('system.theme')
+      ->set('default', 'test_subtheme')
+      ->save();
+    $this->drupalGet('theme-test/prepare-subtheme');
+
+    $this->assertRaw('<p>subThemePrepare: Drupal\theme_test\ThemeTestController::prepareSubtheme<br>
+test_basetheme_theme_prepare_theme_test_prepare<br>
+test_subtheme_theme_prepare_theme_test_prepare<br>
+theme_test_theme_prepare_alter<br>
+test_basetheme_theme_prepare_theme_test_prepare_alter<br>
+test_subtheme_theme_prepare_theme_test_prepare_alter</p>');
+
+    $this->assertRaw('<p>subThemePrepareSuggestion: Drupal\theme_test\ThemeTestController::prepareSubtheme<br>
+theme_test_theme_prepare_theme_test_prepare__implemented<br>
+test_basetheme_theme_prepare_theme_test_prepare<br>
+test_subtheme_theme_prepare_theme_test_prepare<br>
+test_basetheme_theme_prepare_theme_test_prepare__implemented<br>
+test_subtheme_theme_prepare_theme_test_prepare__implemented<br>
+theme_test_theme_prepare_alter<br>
+test_basetheme_theme_prepare_theme_test_prepare_alter<br>
+test_subtheme_theme_prepare_theme_test_prepare_alter<br>
+theme_test_theme_prepare_theme_test_prepare__implemented_alter<br>
+test_basetheme_theme_prepare_theme_test_prepare__implemented_alter<br>
+test_subtheme_theme_prepare_theme_test_prepare__implemented_alter</p>');
   }
 
 }
diff --git a/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php
index 9becb7d..34392d2 100644
--- a/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php
+++ b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php
@@ -185,4 +185,22 @@ function prepare() {
     );
   }
 
+  /**
+   * Menu callback for testing prepare hooks with subthemes.
+   */
+  function prepareSubtheme() {
+    return array(
+      'subThemePrepare' => array(
+        '#theme' => 'theme_test_prepare',
+        '#title' => 'subThemePrepare',
+        '#value' => __METHOD__,
+      ),
+      'subThemePrepareSuggestion' => array(
+        '#theme' => 'theme_test_prepare__implemented',
+        '#title' => 'subThemePrepareSuggestion',
+        '#value' => __METHOD__,
+      ),
+    );
+  }
+
 }
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 e0b2632..0a31b7a 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -228,14 +228,14 @@ function theme_test_theme_prepare_theme_test_prepare__implemented_alter(&$variab
 }
 
 /**
- * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION__SUGGESTION().
+ * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION().
  */
 function theme_test_theme_prepare_theme_test_prepare__implemented__extended(&$variables) {
   $variables['value'] .= "<br>\n" . __FUNCTION__;
 }
 
 /**
- * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION__SUGGESTION_alter().
+ * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION_alter().
  */
 function theme_test_theme_prepare_theme_test_prepare__implemented__extended_alter(&$variables) {
   $variables['value'] .= "<br>\n" . __FUNCTION__;
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.routing.yml b/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
index 3c2f46e..c83536b 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
+++ b/core/modules/system/tests/modules/theme_test/theme_test.routing.yml
@@ -78,15 +78,8 @@ theme_test.prepare:
     _permission: 'access content'
 
 theme_test.prepare_alter:
-  path: '/theme-test/prepare-alter'
+  path: '/theme-test/prepare-subtheme'
   defaults:
-    _content: '\Drupal\theme_test\ThemeTestController::prepareAlter'
-  requirements:
-    _permission: 'access content'
-
-theme_test.prepare_suggestion:
-  path: '/theme-test/prepare-suggestion'
-  defaults:
-    _content: '\Drupal\theme_test\ThemeTestController::prepareSuggestion'
+    _content: '\Drupal\theme_test\ThemeTestController::prepareSubtheme'
   requirements:
     _permission: 'access content'
diff --git a/core/modules/system/tests/themes/test_basetheme/test_basetheme.theme b/core/modules/system/tests/themes/test_basetheme/test_basetheme.theme
new file mode 100644
index 0000000..7def511
--- /dev/null
+++ b/core/modules/system/tests/themes/test_basetheme/test_basetheme.theme
@@ -0,0 +1,34 @@
+<?php
+
+/**
+ * @file
+ * Theme to help test base themes.
+ */
+
+/**
+ * Implements hook_theme_prepare_THEME_HOOK().
+ */
+function test_basetheme_theme_prepare_theme_test_prepare(&$variables) {
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
+}
+
+/**
+ * Implements hook_theme_prepare_THEME_HOOK_alter().
+ */
+function test_basetheme_theme_prepare_theme_test_prepare_alter(&$variables) {
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
+}
+
+/**
+ * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION().
+ */
+function test_basetheme_theme_prepare_theme_test_prepare__implemented(&$variables) {
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
+}
+
+/**
+ * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION_alter().
+ */
+function test_basetheme_theme_prepare_theme_test_prepare__implemented_alter(&$variables) {
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
+}
diff --git a/core/modules/system/tests/themes/test_subtheme/test_subtheme.theme b/core/modules/system/tests/themes/test_subtheme/test_subtheme.theme
new file mode 100644
index 0000000..affb5fb
--- /dev/null
+++ b/core/modules/system/tests/themes/test_subtheme/test_subtheme.theme
@@ -0,0 +1,34 @@
+<?php
+
+/**
+ * @file
+ * Theme to help test subthemes.
+ */
+
+/**
+ * Implements hook_theme_prepare_THEME_HOOK().
+ */
+function test_subtheme_theme_prepare_theme_test_prepare(&$variables) {
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
+}
+
+/**
+ * Implements hook_theme_prepare_THEME_HOOK_alter().
+ */
+function test_subtheme_theme_prepare_theme_test_prepare_alter(&$variables) {
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
+}
+
+/**
+ * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION().
+ */
+function test_subtheme_theme_prepare_theme_test_prepare__implemented(&$variables) {
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
+}
+
+/**
+ * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION_alter().
+ */
+function test_subtheme_theme_prepare_theme_test_prepare__implemented_alter(&$variables) {
+  $variables['value'] .= "<br>\n" . __FUNCTION__;
+}
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.theme b/core/modules/system/tests/themes/test_theme/test_theme.theme
index 99eb007..acf63bd 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.theme
+++ b/core/modules/system/tests/themes/test_theme/test_theme.theme
@@ -105,34 +105,34 @@ function test_theme_theme_prepare_theme_test_prepare(&$variables) {
 /**
  * Implements hook_theme_prepare_THEME_HOOK_alter().
  */
-function test_theme_theme_prepare_theme_test_prepare_alter($variables) {
+function test_theme_theme_prepare_theme_test_prepare_alter(&$variables) {
   $variables['value'] .= "<br>\n" . __FUNCTION__;
 }
 
 /**
  * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION().
  */
-function test_theme_theme_prepare_theme_test_prepare__implemented($variables) {
+function test_theme_theme_prepare_theme_test_prepare__implemented(&$variables) {
   $variables['value'] .= "<br>\n" . __FUNCTION__;
 }
 
 /**
  * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION_alter().
  */
-function test_theme_theme_prepare_theme_test_prepare__implemented_alter($variables) {
+function test_theme_theme_prepare_theme_test_prepare__implemented_alter(&$variables) {
   $variables['value'] .= "<br>\n" . __FUNCTION__;
 }
 
 /**
- * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION__SUGGESTION().
+ * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION().
  */
-function test_theme_theme_prepare_theme_test_prepare__implemented__extended($variables) {
+function test_theme_theme_prepare_theme_test_prepare__implemented__extended(&$variables) {
   $variables['value'] .= "<br>\n" . __FUNCTION__;
 }
 
 /**
- * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION__SUGGESTION_alter().
+ * Implements hook_theme_prepare_THEME_HOOK__SUGGESTION_alter().
  */
-function test_theme_theme_prepare_theme_test_prepare__implemented__extended_alter($variables) {
+function test_theme_theme_prepare_theme_test_prepare__implemented__extended_alter(&$variables) {
   $variables['value'] .= "<br>\n" . __FUNCTION__;
 }