diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 8c60084..3459249 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -115,7 +115,7 @@ function drupal_theme_rebuild() {
  */
 function drupal_find_theme_functions($cache, $prefixes) {
   $implementations = [];
-  $grouped_functions = drupal_group_functions_by_prefix();
+  $grouped_functions = \Drupal::service('theme.registry')->getPrefixGroupedUserFunctions();
 
   foreach ($cache as $hook => $info) {
     foreach ($prefixes as $prefix) {
@@ -162,25 +162,6 @@ function drupal_find_theme_functions($cache, $prefixes) {
 }
 
 /**
- * Group all user functions by word before first underscore.
- *
- * @return array
- *   Functions grouped by the first prefix.
- */
-function drupal_group_functions_by_prefix() {
-  $functions = get_defined_functions();
-
-  $grouped_functions = [];
-  // Splitting user defined functions into groups by the first prefix.
-  foreach ($functions['user'] as $function) {
-    list($first_prefix,) = explode('_', $function, 2);
-    $grouped_functions[$first_prefix][] = $function;
-  }
-
-  return $grouped_functions;
-}
-
-/**
  * Allows themes and/or theme engines to easily discover overridden templates.
  *
  * @param $cache
diff --git a/core/lib/Drupal/Core/Render/theme.api.php b/core/lib/Drupal/Core/Render/theme.api.php
index 689dce0..e2b7624 100644
--- a/core/lib/Drupal/Core/Render/theme.api.php
+++ b/core/lib/Drupal/Core/Render/theme.api.php
@@ -99,10 +99,12 @@
  * before the template file is invoked to modify the variables that are passed
  * to the template. These make up the "preprocessing" phase, and are executed
  * (if they exist), in the following order (note that in the following list,
- * HOOK indicates the theme hook name, MODULE indicates a module name, THEME
- * indicates a theme name, and ENGINE indicates a theme engine name). Modules,
- * themes, and theme engines can provide these functions to modify how the
- * data is preprocessed, before it is passed to the theme template:
+ * HOOK indicates the hook being called or a less specific hook. For example, if
+ * '#theme' => 'node__article' is called, hook is node__article and node. MODULE
+ * indicates a module name, THEME indicates a theme name, and ENGINE indicates a
+ * theme engine name). Modules, themes, and theme engines can provide these
+ * functions to modify how the data is preprocessed, before it is passed to the
+ * theme template:
  * - template_preprocess(&$variables, $hook): Creates a default set of variables
  *   for all theme hooks with template implementations. Provided by Drupal Core.
  * - template_preprocess_HOOK(&$variables): Should be implemented by the module
diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index bb89707..8902daa 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -342,9 +342,12 @@ protected function build() {
       $this->processExtension($cache, $this->theme->getEngine(), 'theme_engine', $this->theme->getName(), $this->theme->getPath());
     }
 
-    // Finally, hooks provided by the theme itself.
+    // Hooks provided by the theme itself.
     $this->processExtension($cache, $this->theme->getName(), 'theme', $this->theme->getName(), $this->theme->getPath());
 
+    // Discover and add all preprocess functions for theme hook suggestions.
+    $this->postProcessExtension($cache, $this->theme);
+
     // Let modules and themes alter the registry.
     $this->moduleHandler->alter('theme_registry', $cache);
     $this->themeManager->alterForTheme($this->theme, 'theme_registry', $cache);
@@ -420,7 +423,7 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
       'base hook' => TRUE,
     );
 
-    $module_list = array_keys((array) $this->moduleHandler->getModuleList());
+    $module_list = array_keys($this->moduleHandler->getModuleList());
 
     // Invoke the hook_theme() implementation, preprocess what is returned, and
     // merge it into $cache.
@@ -438,6 +441,12 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
         $result[$hook]['type'] = $type;
         $result[$hook]['theme path'] = $path;
 
+        // If a theme hook has a base hook, mark its preprocess functions always
+        // incomplete in order to inherit the base hook's preprocess functions.
+        if (!empty($result[$hook]['base hook'])) {
+          $result[$hook]['incomplete preprocess functions'] = TRUE;
+        }
+
         if (isset($cache[$hook]['includes'])) {
           $result[$hook]['includes'] = $cache[$hook]['includes'];
         }
@@ -556,14 +565,142 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
             $cache[$hook]['preprocess functions'][] = $name . '_preprocess_' . $hook;
             $cache[$hook]['theme path'] = $path;
           }
-          // Ensure uniqueness.
-          $cache[$hook]['preprocess functions'] = array_unique($cache[$hook]['preprocess functions']);
         }
       }
     }
   }
 
   /**
+   * Completes the definition of the requested suggestion hook.
+   *
+   * @param string $hook
+   *   The name of the suggestion hook to complete.
+   * @param array $cache
+   *   The theme registry, as documented in
+   *   \Drupal\Core\Theme\Registry::processExtension().
+   */
+  protected function completeSuggestion($hook, array &$cache) {
+    $previous_hook = $hook;
+    $incomplete_previous_hook = array();
+    while ((!isset($cache[$previous_hook]) || isset($cache[$previous_hook]['incomplete preprocess functions']))
+      && $pos = strrpos($previous_hook, '__')) {
+      if (isset($cache[$previous_hook]) && !$incomplete_previous_hook && isset($cache[$previous_hook]['incomplete preprocess functions'])) {
+        $incomplete_previous_hook = $cache[$previous_hook];
+        unset($incomplete_previous_hook['incomplete preprocess functions']);
+      }
+      $previous_hook = substr($previous_hook, 0, $pos);
+
+      // If base hook exists clone of it for the preprocess function
+      // without a template.
+      // @see https://www.drupal.org/node/2457295
+      if (isset($cache[$previous_hook]) && !isset($cache[$previous_hook]['incomplete preprocess functions'])) {
+        $cache[$hook] = $incomplete_previous_hook + $cache[$previous_hook];
+        if (isset($incomplete_previous_hook['preprocess functions'])) {
+          $diff = array_diff($incomplete_previous_hook['preprocess functions'], $cache[$previous_hook]['preprocess functions']);
+          $cache[$hook]['preprocess functions'] = array_merge($cache[$previous_hook]['preprocess functions'], $diff);
+        }
+        // If a base hook isn't set, this is the actual base hook.
+        if (!isset($cache[$previous_hook]['base hook'])) {
+          $cache[$hook]['base hook'] = $previous_hook;
+        }
+      }
+    }
+  }
+
+  /**
+   * Completes the theme registry adding discovered functions and hooks.
+   *
+   * @param array $cache
+   *   The theme registry as documented in
+   *   \Drupal\Core\Theme\Registry::processExtension().
+   * @param \Drupal\Core\Theme\ActiveTheme $theme
+   *   Current active theme.
+   *
+   * @see ::processExtension()
+   */
+  protected function postProcessExtension(array &$cache, ActiveTheme $theme) {
+    $grouped_functions = $this->getPrefixGroupedUserFunctions();
+
+    // Gather prefixes. This will be used to limit the found functions to the
+    // expected naming conventions.
+    $prefixes = array_keys((array) $this->moduleHandler->getModuleList());
+    foreach (array_reverse($theme->getBaseThemes()) as $base) {
+      $prefixes[] = $base->getName();
+    }
+    if ($theme->getEngine()) {
+      $prefixes[] = $theme->getEngine() . '_engine';
+    }
+    $prefixes[] = $theme->getName();
+
+    // Collect all variable preprocess functions in the correct order.
+    $suggestion_level = [];
+    $matches = [];
+    // Look for functions named according to the pattern and add them if they
+    // have matching hooks in the registry.
+    foreach ($prefixes as $prefix) {
+      // Grep only the functions which are within the prefix group.
+      list($first_prefix,) = explode('_', $prefix, 2);
+      if (!isset($grouped_functions[$first_prefix])) {
+        continue;
+      }
+      // Add the function and the name of the associated theme hook to the list
+      // of preprocess functions grouped by suggestion specificity if a matching
+      // base hook is found.
+      foreach ($grouped_functions[$first_prefix] as $candidate) {
+        if (preg_match("/^{$prefix}_preprocess_(((?:[^_]++|_(?!_))+)__.*)/", $candidate, $matches)) {
+          if (isset($cache[$matches[2]])) {
+            $level = substr_count($matches[1], '__');
+            $suggestion_level[$level][$candidate] = $matches[1];
+          }
+        }
+      }
+    }
+
+    // Add missing variable preprocessors. This is needed for modules that do
+    // not explicitly register the hook. For example, when a theme contains a
+    // variable preprocess function but it does not implement a template, it
+    // will go missing. This will add the expected function. It also allows
+    // modules or themes to have a variable process function based on a pattern
+    // even if the hook does not exist.
+    for ($level = 1; $level <= count($suggestion_level); $level++) {
+      foreach ($suggestion_level[$level] as $preprocessor => $hook) {
+        if (isset($cache[$hook]['preprocess functions']) && !in_array($hook, $cache[$hook]['preprocess functions'])) {
+          // Add missing preprocessor to existing hook.
+          $cache[$hook]['preprocess functions'][] = $preprocessor;
+        }
+        elseif (!isset($cache[$hook]) && strpos($hook, '__')) {
+          // Process non-existing hook and register it.
+          // Look for a previously defined hook that is either a less specific
+          // suggestion hook or the base hook.
+          $this->completeSuggestion($hook, $cache);
+          $cache[$hook]['preprocess functions'][] = $preprocessor;
+        }
+      }
+    }
+    // Inherit all base hook variable preprocess functions into suggestion
+    // hooks. This ensures that derivative hooks have a complete set of variable
+    // preprocess functions.
+    foreach ($cache as $hook => $info) {
+      // The 'base hook' is only applied to derivative hooks already registered
+      // from a pattern. This is typically set from
+      // drupal_find_theme_functions() and drupal_find_theme_templates().
+      if (isset($info['incomplete preprocess functions'])) {
+        $this->completeSuggestion($hook, $cache);
+        unset($cache[$hook]['incomplete preprocess functions']);
+      }
+
+      // Optimize the registry.
+      if (isset($cache[$hook]['preprocess functions']) && empty($cache[$hook]['preprocess functions'])) {
+        unset($cache[$hook]['preprocess functions']);
+      }
+      // Ensure uniqueness.
+      if (isset($cache[$hook]['preprocess functions'])) {
+        $cache[$hook]['preprocess functions'] = array_unique($cache[$hook]['preprocess functions']);
+      }
+    }
+  }
+
+  /**
    * Invalidates theme registry caches.
    *
    * To be called when the list of enabled extensions is changed.
@@ -590,6 +727,25 @@ public function destruct() {
   }
 
   /**
+   * Gets all user functions grouped by the word before the first underscore.
+   *
+   * @return array
+   *   Functions grouped by the first prefix.
+   */
+  public function getPrefixGroupedUserFunctions() {
+    $functions = get_defined_functions();
+
+    $grouped_functions = [];
+    // Splitting user defined functions into groups by the first prefix.
+    foreach ($functions['user'] as $function) {
+      list($first_prefix,) = explode('_', $function, 2);
+      $grouped_functions[$first_prefix][] = $function;
+    }
+
+    return $grouped_functions;
+  }
+
+  /**
    * Wraps drupal_get_path().
    *
    * @param string $module
diff --git a/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
index 31c10a7..5a2af2e 100644
--- a/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -279,12 +279,10 @@ public function render($hook, array $variables) {
           include_once $this->root . '/' . $include_file;
         }
       }
-      // Replace the preprocess functions with those from the base hook.
       if (isset($base_hook_info['preprocess functions'])) {
         // Set a variable for the 'theme_hook_suggestion'. This is used to
         // maintain backwards compatibility with template engines.
         $theme_hook_suggestion = $hook;
-        $info['preprocess functions'] = $base_hook_info['preprocess functions'];
       }
     }
     if (isset($info['preprocess functions'])) {
diff --git a/core/modules/system/src/Tests/Theme/RegistryTest.php b/core/modules/system/src/Tests/Theme/RegistryTest.php
index 8a72ecc..e6c80be 100644
--- a/core/modules/system/src/Tests/Theme/RegistryTest.php
+++ b/core/modules/system/src/Tests/Theme/RegistryTest.php
@@ -99,6 +99,46 @@ public function testMultipleSubThemes() {
       'template_preprocess',
       'test_basetheme_preprocess_theme_test_template_test',
     ], $preprocess_functions);
+
+  }
+
+  /**
+   * Tests the theme registry with suggestions.
+   */
+  public function testSuggestionPreprocessFunctions() {
+    $theme_handler = \Drupal::service('theme_handler');
+    $theme_handler->install(['test_theme']);
+
+    $registry_theme = new Registry(\Drupal::root(), \Drupal::cache(), \Drupal::lock(), \Drupal::moduleHandler(), $theme_handler, \Drupal::service('theme.initialization'), 'test_theme');
+    $registry_theme->setThemeManager(\Drupal::theme());
+
+    $suggestions = ['__kitten', '__flamingo'];
+    $expected_preprocess_functions = [
+      'template_preprocess',
+      'theme_test_preprocess_theme_test_preprocess_suggestions',
+    ];
+    $suggestion = '';
+    $hook = 'theme_test_preprocess_suggestions';
+    do {
+      $hook .= "$suggestion";
+      $expected_preprocess_functions[] = "test_theme_preprocess_$hook";
+      $preprocess_functions = $registry_theme->get()[$hook]['preprocess functions'];
+      $this->assertIdentical($expected_preprocess_functions, $preprocess_functions, "$hook has correct preprocess functions.");
+    } while ($suggestion = array_shift($suggestions));
+
+    $expected_preprocess_functions = [
+      'template_preprocess',
+      'theme_test_preprocess_theme_test_preprocess_suggestions',
+      'test_theme_preprocess_theme_test_preprocess_suggestions',
+      'test_theme_preprocess_theme_test_preprocess_suggestions__kitten',
+    ];
+
+    $preprocess_functions = $registry_theme->get()['theme_test_preprocess_suggestions__kitten__meerkat']['preprocess functions'];
+    $this->assertIdentical($expected_preprocess_functions, $preprocess_functions, 'Suggestion implemented as a function correctly inherits preprocess functions.');
+
+    $preprocess_functions = $registry_theme->get()['theme_test_preprocess_suggestions__kitten__bearcat']['preprocess functions'];
+    $this->assertIdentical($expected_preprocess_functions, $preprocess_functions, 'Suggestion implemented as a template correctly inherits preprocess functions.');
+
   }
 
   /**
diff --git a/core/modules/system/src/Tests/Theme/ThemeTest.php b/core/modules/system/src/Tests/Theme/ThemeTest.php
index c2c0e0b..9db4fa8 100644
--- a/core/modules/system/src/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/src/Tests/Theme/ThemeTest.php
@@ -277,7 +277,7 @@ function testPreprocessHtml() {
   /**
    * Tests that region attributes can be manipulated via preprocess functions.
    */
-  function testRegionClass() {
+  public function testRegionClass() {
     \Drupal::service('module_installer')->install(array('block', 'theme_region_test'));
 
     // Place a block.
@@ -287,4 +287,31 @@ function testRegionClass() {
     $this->assertEqual(count($elements), 1, 'New class found.');
   }
 
+  /**
+   * Ensures suggestion preprocess functions run 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.
+   */
+  public function testSuggestionPreprocessForDefaults() {
+    \Drupal::service('theme_handler')->setDefault('test_theme');
+    // Test with both an unprimed and primed theme registry.
+    drupal_theme_rebuild();
+    for ($i = 0; $i < 2; $i++) {
+      $this->drupalGet('theme-test/preprocess-suggestions');
+      $items = $this->cssSelect('.suggestion');
+      $expected_values = [
+        'Suggestion',
+        'Kitten',
+        'Monkey',
+        'Kitten',
+        'Flamingo',
+      ];
+      foreach ($expected_values as $key => $value) {
+        $this->assertEqual((string) $value, $items[$key]);
+      }
+    }
+  }
+
 }
diff --git a/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php b/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
index a8f16c6..68b627e 100644
--- a/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
+++ b/core/modules/system/tests/modules/theme_test/src/ThemeTestController.php
@@ -143,4 +143,25 @@ public function nonHtml() {
     return new JsonResponse(['theme_initialized' => $theme_initialized]);
   }
 
+  /**
+   * Controller for testing preprocess functions with theme suggestions.
+   */
+  public function preprocessSuggestions() {
+    return [
+      [
+        '#theme' => 'theme_test_preprocess_suggestions',
+        '#foo' => 'suggestion',
+      ],
+      [
+        '#theme' => 'theme_test_preprocess_suggestions',
+        '#foo' => 'kitten',
+      ],
+      [
+        '#theme' => 'theme_test_preprocess_suggestions',
+        '#foo' => 'monkey',
+      ],
+      ['#theme' => 'theme_test_preprocess_suggestions__kitten__flamingo'],
+    ];
+  }
+
 }
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme-test-preprocess-suggestions.html.twig b/core/modules/system/tests/modules/theme_test/templates/theme-test-preprocess-suggestions.html.twig
new file mode 100644
index 0000000..512613d
--- /dev/null
+++ b/core/modules/system/tests/modules/theme_test/templates/theme-test-preprocess-suggestions.html.twig
@@ -0,0 +1,4 @@
+<div class="suggestion">{{ foo }}</div>
+{% if bar %}
+  <div class="suggestion">{{ bar }}</div>
+{% endif %}
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 7ba5c2d..8b820c3 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -55,6 +55,12 @@ function theme_test_theme($existing, $type, $theme, $path) {
   $info['test_theme_not_existing_function'] = array(
     'function' => 'test_theme_not_existing_function',
   );
+  $items['theme_test_preprocess_suggestions'] = [
+    'variables' => [
+      'foo' => '',
+      'bar' => '',
+    ],
+  ];
   return $items;
 }
 
@@ -90,6 +96,27 @@ function theme_theme_test_function_template_override($variables) {
 }
 
 /**
+ * Implements hook_theme_suggestions_HOOK().
+ */
+function theme_test_theme_suggestions_theme_test_preprocess_suggestions($variables) {
+  return ['theme_test_preprocess_suggestions__' . $variables['foo']];
+}
+
+/**
+ * Implements hook_preprocess_HOOK().
+ */
+function theme_test_preprocess_theme_test_preprocess_suggestions(&$variables) {
+  $variables['foo'] = 'Theme hook implementor=theme_theme_test_preprocess_suggestions().';
+}
+
+/**
+ * Tests a module overriding a default hook with a suggestion.
+ */
+function theme_test_preprocess_theme_test_preprocess_suggestions__monkey(&$variables) {
+  $variables['foo'] = 'Monkey';
+}
+
+/**
  * Prepares variables for test render element templates.
  *
  * Default template: theme-test-render-element.html.twig.
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 2dbd188..1ff61cf 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
@@ -103,3 +103,10 @@ theme_test.non_html:
     _controller: '\Drupal\theme_test\ThemeTestController::nonHtml'
   requirements:
     _access: 'TRUE'
+
+theme_test.preprocess_suggestions:
+  path: '/theme-test/preprocess-suggestions'
+  defaults:
+    _controller: '\Drupal\theme_test\ThemeTestController::preprocessSuggestions'
+  requirements:
+    _access: 'TRUE'
diff --git a/core/modules/system/tests/themes/test_theme/templates/theme-test-preprocess-suggestions--kitten--bearcat.html.twig b/core/modules/system/tests/themes/test_theme/templates/theme-test-preprocess-suggestions--kitten--bearcat.html.twig
new file mode 100644
index 0000000..512613d
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/templates/theme-test-preprocess-suggestions--kitten--bearcat.html.twig
@@ -0,0 +1,4 @@
+<div class="suggestion">{{ foo }}</div>
+{% if bar %}
+  <div class="suggestion">{{ bar }}</div>
+{% endif %}
diff --git a/core/modules/system/tests/themes/test_theme/templates/theme-test-preprocess-suggestions--suggestion.html.twig b/core/modules/system/tests/themes/test_theme/templates/theme-test-preprocess-suggestions--suggestion.html.twig
new file mode 100644
index 0000000..e77924d
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme/templates/theme-test-preprocess-suggestions--suggestion.html.twig
@@ -0,0 +1 @@
+<div class="suggestion">{{ foo }}</div>
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 9350755..3ce72e3 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.theme
+++ b/core/modules/system/tests/themes/test_theme/test_theme.theme
@@ -105,3 +105,40 @@ function test_theme_theme_test_function_suggestions__module_override($variables)
 function test_theme_theme_registry_alter(&$registry) {
   $registry['theme_test_template_test']['variables']['additional'] = 'value';
 }
+
+/**
+ * Tests a theme overriding a suggestion of a base theme hook.
+ */
+function test_theme_theme_test_preprocess_suggestions__kitten__meerkat($variables) {
+  return 'Theme hook implementor=test_theme_theme_test__suggestion(). Foo=' . $variables['foo'];
+}
+
+/**
+ * Tests a theme overriding a default hook with a suggestion.
+ *
+ * Implements hook_preprocess_HOOK().
+ */
+function test_theme_preprocess_theme_test_preprocess_suggestions(&$variables) {
+  $variables['foo'] = 'Theme hook implementor=test_theme_preprocess_theme_test_preprocess_suggestions().';
+}
+
+/**
+ * Tests a theme overriding a default hook with a suggestion.
+ */
+function test_theme_preprocess_theme_test_preprocess_suggestions__suggestion(&$variables) {
+  $variables['foo'] = 'Suggestion';
+}
+
+/**
+ * Tests a theme overriding a default hook with a suggestion.
+ */
+function test_theme_preprocess_theme_test_preprocess_suggestions__kitten(&$variables) {
+  $variables['foo'] = 'Kitten';
+}
+
+/**
+ * Tests a theme overriding a default hook with a suggestion.
+ */
+function test_theme_preprocess_theme_test_preprocess_suggestions__kitten__flamingo(&$variables) {
+  $variables['bar'] = 'Flamingo';
+}
diff --git a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
index 4a9101d..0cb5a25 100644
--- a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
+++ b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
@@ -105,6 +105,9 @@ public function testGetRegistryForModule() {
       ->method('getImplementations')
       ->with('theme')
       ->will($this->returnValue(array('theme_test')));
+    $this->moduleHandler->expects($this->atLeastOnce())
+      ->method('getModuleList')
+      ->willReturn([]);
 
     $registry = $this->registry->get();