diff --git a/core/lib/Drupal/Core/Controller/ControllerResolver.php b/core/lib/Drupal/Core/Controller/ControllerResolver.php
index a8f2b3c..c0e4b5b 100644
--- a/core/lib/Drupal/Core/Controller/ControllerResolver.php
+++ b/core/lib/Drupal/Core/Controller/ControllerResolver.php
@@ -64,7 +64,7 @@ public function __construct(HttpMessageFactoryInterface $http_message_factory, C
    * {@inheritdoc}
    */
   public function getControllerFromDefinition($controller, $path = '') {
-    if (is_array($controller) || (is_object($controller) && method_exists($controller, '__invoke'))) {
+    if (is_array($controller) || (is_object($controller) && method_exists($controller, '__invoke')) || function_exists($controller)) {
       return $controller;
     }
 
diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index e2a89a9..cc8a938 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -429,7 +429,15 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
     // Invoke the hook_theme() implementation, preprocess what is returned, and
     // merge it into $cache.
     $function = $name . '_theme';
-    if (function_exists($function)) {
+
+    $theme_engine_service = NULL;
+    if ($type == 'theme_engine' && \Drupal::service('theme_engine.' . $name)) {
+      /** @var \Drupal\Core\Theme\ThemeEngineInterface|null $theme_engine_service */
+      $theme_engine_service = \Drupal::hasService('theme_engine.' . $name) ? \Drupal::service('theme_engine.' . $name) : NULL;
+      $function = [$theme_engine_service, 'theme'];
+    }
+
+    if (is_callable($function)) {
       $result = $function($cache, $type, $theme, $path);
       foreach ($result as $hook => $info) {
         // When a theme or engine overrides a module's theme function
@@ -572,6 +580,9 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
           if (isset($info['template']) && function_exists($name . '_preprocess')) {
             $cache[$hook]['preprocess functions'][] = $name . '_preprocess';
           }
+          if (isset($info['template']) && $type == 'theme_engine' && $theme_engine_service) {
+            $cache[$hook]['preprocess functions'][] = "theme_engine.$name:preprocess";
+          }
           if (function_exists($name . '_preprocess_' . $hook)) {
             $cache[$hook]['preprocess functions'][] = $name . '_preprocess_' . $hook;
             $cache[$hook]['theme path'] = $path;
diff --git a/core/lib/Drupal/Core/Theme/ThemeEngineCollection.php b/core/lib/Drupal/Core/Theme/ThemeEngineCollection.php
new file mode 100644
index 0000000..1db96f0
--- /dev/null
+++ b/core/lib/Drupal/Core/Theme/ThemeEngineCollection.php
@@ -0,0 +1,30 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Theme\ThemeEngineCollection.
+ */
+
+namespace Drupal\Core\Theme;
+
+class ThemeEngineCollection {
+
+  /** @var  ThemeEngineInterface[] */
+  protected $engines;
+
+  protected $serviceIDs;
+
+  public function addThemeEngine($name, ThemeEngineInterface $theme_engine, $service_id) {
+    $this->engines[$name] = $theme_engine;
+    $this->serviceIDs[$name] = $service_id;
+  }
+
+  public function getThemeEngine($name) {
+    return isset($this->engines[$name]) ? $this->engines[$name] : NULL;
+  }
+
+  public function getServiceID($name) {
+    return isset($this->serviceIDs[$name]) ? $this->serviceIDs[$name] : NULL;
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Theme/ThemeEngineInterface.php b/core/lib/Drupal/Core/Theme/ThemeEngineInterface.php
new file mode 100644
index 0000000..4c507e2
--- /dev/null
+++ b/core/lib/Drupal/Core/Theme/ThemeEngineInterface.php
@@ -0,0 +1,60 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Theme\ThemeEngineInterface.
+ */
+
+namespace Drupal\Core\Theme;
+
+use Drupal\Core\Extension\Extension;
+
+/**
+ * Defines an interface for a theme engine.
+ *
+ * A theme engine controls how to render a render a template.
+ */
+interface ThemeEngineInterface {
+
+  /**
+   * Initializes a theme.
+   *
+   * The theme engine might load for example a .theme file to "load" a theme.
+   *
+   * @param \Drupal\Core\Extension\Extension $theme
+   *   The theme to initialize.
+   */
+  public function initTheme(Extension $theme);
+
+  /**
+   * Wraps hook_theme() for engines.
+   */
+  public function theme($existing, $type, $theme, $path);
+
+  /**
+   * Returns the extension used by the theme engine.
+   *
+   * @return string
+   *   The extension.
+   */
+  public function getExtension();
+
+  /**
+   * Renders a template.
+   *
+   * @param string $template_file
+   *   The file name of the template to render.
+   * @param array $variables
+   *   A keyed array of variables that will appear in the output.
+   *
+   * @retur string
+   *   The output generated by the template, plus any debug information.
+   */
+  public function renderTemplate($template_file, $variables);
+
+  /**
+   * Wraps a preprocess function for engines.
+   */
+  public function preprocess();
+
+}
diff --git a/core/lib/Drupal/Core/Theme/ThemeInitialization.php b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
index 88c06e2..2c84137 100644
--- a/core/lib/Drupal/Core/Theme/ThemeInitialization.php
+++ b/core/lib/Drupal/Core/Theme/ThemeInitialization.php
@@ -137,7 +137,15 @@ public function loadActiveTheme(ActiveTheme $active_theme) {
       // Include the engine.
       include_once $this->root . '/' . $active_theme->getOwner();
 
-      if (function_exists($theme_engine . '_init')) {
+      /** @var \Drupal\Core\Theme\ThemeEngineInterface|null $theme_engine_service */
+      $theme_engine_service = \Drupal::hasService('theme_engine.' . $theme_engine) ? \Drupal::service('theme_engine.' . $theme_engine) : NULL;
+      if ($theme_engine_service) {
+        foreach ($active_theme->getBaseThemes() as $base) {
+          $theme_engine_service->initTheme($base->getExtension());
+        }
+        $theme_engine_service->initTheme($active_theme->getExtension());
+      }
+      elseif (function_exists($theme_engine . '_init')) {
         foreach ($active_theme->getBaseThemes() as $base) {
           call_user_func($theme_engine . '_init', $base->getExtension());
         }
diff --git a/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
index d4c6a2c..16c5ecd 100644
--- a/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -8,6 +8,7 @@
 namespace Drupal\Core\Theme;
 
 use Drupal\Component\Render\MarkupInterface;
+use Drupal\Core\Controller\ControllerResolverInterface;
 use Drupal\Core\Render\Markup;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\Routing\StackedRouteMatchInterface;
@@ -61,6 +62,13 @@ class ThemeManager implements ThemeManagerInterface {
   protected $root;
 
   /**
+   * The controller resolver.
+   *
+   * @var \Drupal\Core\Controller\ControllerResolverInterface
+   */
+  protected $controllerResolver;
+
+  /**
    * Constructs a new ThemeManager object.
    *
    * @param string $root
@@ -72,13 +80,16 @@ class ThemeManager implements ThemeManagerInterface {
    * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
    *   The request stack.
    * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   * @param \Drupal\Core\Controller\ControllerResolverInterface $controller_resolver
+   *   The controller resolver.
    */
-  public function __construct($root, ThemeNegotiatorInterface $theme_negotiator, ThemeInitializationInterface $theme_initialization, RequestStack $request_stack, ModuleHandlerInterface $module_handler) {
+  public function __construct($root, ThemeNegotiatorInterface $theme_negotiator, ThemeInitializationInterface $theme_initialization, RequestStack $request_stack, ModuleHandlerInterface $module_handler, ControllerResolverInterface $controller_resolver = NULL) {
     $this->root = $root;
     $this->themeNegotiator = $theme_negotiator;
     $this->themeInitialization = $theme_initialization;
     $this->requestStack = $request_stack;
     $this->moduleHandler = $module_handler;
+    $this->controllerResolver = $controller_resolver ?: \Drupal::service('controller_resolver');
   }
 
   /**
@@ -289,8 +300,11 @@ public function render($hook, array $variables) {
     }
     if (isset($info['preprocess functions'])) {
       foreach ($info['preprocess functions'] as $preprocessor_function) {
-        if (function_exists($preprocessor_function)) {
-          $preprocessor_function($variables, $hook, $info);
+        // Allow preprocess functions from services.
+        if (strpos($preprocessor_function, ':') === FALSE || $preprocessor_function = $this->controllerResolver->getControllerFromDefinition($preprocessor_function)) {
+          if (is_callable($preprocessor_function)) {
+            $preprocessor_function($variables, $hook, $info);
+          }
         }
       }
       // Allow theme preprocess functions to set $variables['#attached'] and
@@ -332,10 +346,19 @@ public function render($hook, array $variables) {
       $theme_engine = $active_theme->getEngine();
       if (isset($theme_engine)) {
         if ($info['type'] != 'module') {
-          if (function_exists($theme_engine . '_render_template')) {
+          $theme_engine_service = \Drupal::hasService('theme_engine.' . $theme_engine) ? \Drupal::service('theme_engine.' . $theme_engine) : NULL;
+          if ($theme_engine_service) {
+            $render_function = [$theme_engine_service, 'renderTemplate'];
+          }
+          elseif (function_exists($theme_engine . '_render_template')) {
             $render_function = $theme_engine . '_render_template';
           }
-          $extension_function = $theme_engine . '_extension';
+          if ($theme_engine_service) {
+            $extension_function = [$theme_engine_service, 'getExtension'];
+          }
+          else {
+            $extension_function = $theme_engine . '_extension';
+          }
           if (function_exists($extension_function)) {
             $extension = $extension_function();
           }
diff --git a/core/modules/system/tests/themes/engines/nyan_cat/nyan_cat.engine b/core/modules/system/tests/themes/engines/nyan_cat/nyan_cat.engine
deleted file mode 100644
index 04666dc..0000000
--- a/core/modules/system/tests/themes/engines/nyan_cat/nyan_cat.engine
+++ /dev/null
@@ -1,55 +0,0 @@
-<?php
-
-/**
- * @file
- * Handles integration of Nyan cat templates because we love kittens.
- */
-
-use Drupal\Core\Extension\Extension;
-
-/**
- * Includes .theme file from themes.
- *
- * @param \Drupal\Core\Extension\Extension $theme
- *   The theme extension object.
- */
-function nyan_cat_init(Extension $theme) {
-  $theme->load();
-}
-
-/**
- * Implements hook_theme().
- */
-function nyan_cat_theme($existing, $type, $theme, $path) {
-  $templates = drupal_find_theme_functions($existing, array($theme));
-  $templates += drupal_find_theme_templates($existing, '.nyan-cat.html', $path);
-  return $templates;
-}
-
-/**
- * Implements hook_extension().
- */
-function nyan_cat_extension() {
-  return '.nyan-cat.html';
-}
-
-/**
- * Implements hook_render_template().
- *
- * @param string $template_file
- *   The filename of the template to render.
- * @param mixed[] $variables
- *   A keyed array of variables that will appear in the output.
- *
- * @return string
- *   The output generated by the template.
- */
-function nyan_cat_render_template($template_file, $variables) {
-  $output = str_replace('div', 'nyancat', file_get_contents(\Drupal::root() . '/' . $template_file));
-  foreach ($variables as $key => $variable) {
-    if (strpos($output, '9' . $key) !== FALSE) {
-      $output = str_replace('9' . $key, theme_render_and_autoescape($variable), $output);
-    }
-  }
-  return $output;
-}
diff --git a/core/modules/system/tests/themes/engines/nyan_cat/nyat_cat.services.yml b/core/modules/system/tests/themes/engines/nyan_cat/nyat_cat.services.yml
new file mode 100644
index 0000000..bac750c
--- /dev/null
+++ b/core/modules/system/tests/themes/engines/nyan_cat/nyat_cat.services.yml
@@ -0,0 +1,2 @@
+theme_engine.nyan_cat:
+  class: Drupal\nyan_cat\NyanCatThemeEngine
diff --git a/core/modules/system/tests/themes/engines/nyan_cat/src/NyanCatThemeEngine.php b/core/modules/system/tests/themes/engines/nyan_cat/src/NyanCatThemeEngine.php
new file mode 100644
index 0000000..f3a682e
--- /dev/null
+++ b/core/modules/system/tests/themes/engines/nyan_cat/src/NyanCatThemeEngine.php
@@ -0,0 +1,63 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\nyan_cat\NyanCatThemeEngine.
+ */
+
+namespace Drupal\nyan_cat;
+
+use Drupal\Core\Extension\Extension;
+use Drupal\Core\Theme\ThemeEngineInterface;
+
+/**
+ * NYAN NYAN NYAN!
+ *
+ * If you don't understand yet what nyan cat is, please look at
+ * https://www.youtube.com/watch?v=IAISUDbjXj0
+ */
+class NyanCatThemeEngine implements ThemeEngineInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function initTheme(Extension $theme) {
+    $theme->load();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function theme($existing, $type, $theme, $path) {
+    $templates = drupal_find_theme_functions($existing, array($theme));
+    $templates += drupal_find_theme_templates($existing, '.nyan-cat.html', $path);
+    return $templates;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getExtension() {
+    return '.nyan-cat.html';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function renderTemplate($template_file, $variables) {
+    $output = str_replace('div', 'nyancat', file_get_contents(\Drupal::root() . '/' . $template_file));
+    foreach ($variables as $key => $variable) {
+      if (strpos($output, '9' . $key) !== FALSE) {
+        $output = str_replace('9' . $key, theme_render_and_autoescape($variable), $output);
+      }
+    }
+    return $output;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function preprocess() {
+  }
+
+}