diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index a38dbe0..80e6144 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -269,7 +269,7 @@ function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callb
   // @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 .twig conversion.
+  // layer to allow us to perform core-wide .tpl.php to .html.twig conversion.
   include_once DRUPAL_ROOT . '/core/themes/engines/twig/twig.engine';
 
   if (isset($registry_callback)) {
@@ -514,7 +514,7 @@ function _theme_process_registry(&$cache, $name, $type, $theme, $path) {
           // Add two render engines for modules.
           // @todo Remove and make twig the default engine.
           $render_engines = array(
-            '.twig' => 'twig',
+            '.html.twig' => 'twig',
             '.tpl.php' => 'phptemplate'
           );
 
@@ -1291,10 +1291,10 @@ function drupal_find_theme_templates($cache, $extension, $path) {
     if (strpos($file->uri, str_replace($subtheme_paths, '', $file->uri)) !== 0) {
       continue;
     }
-    // Chop off the remaining '.tpl' extension. $template already has the
-    // rightmost extension removed, but there might still be more, such as with
-    // .tpl.php, which still has .tpl in $template at this point.
-    if (($pos = strpos($template, '.tpl')) !== FALSE) {
+    // Chop off the remaining extensions if there are any. $template already
+    // has the rightmost extension removed, but there might still be more,
+    // such as with .tpl.php, which still has .tpl in $template at this point.
+    if (($pos = strrpos($template, '.')) !== FALSE) {
       $template = substr($template, 0, $pos);
     }
     // Transform - in filenames to _ to match function naming scheme
@@ -1341,7 +1341,7 @@ function drupal_find_theme_templates($cache, $extension, $path) {
           // already has the rightmost extension removed, but there might still
           // be more, such as with .tpl.php, which still has .tpl in $template
           // at this point.
-          if (($pos = strpos($match, '.')) !== FALSE) {
+          if (($pos = strrpos($match, '.')) !== FALSE) {
             $file = substr($match, 0, $pos);
           }
           // Put the underscores back in for the hook name and register this
diff --git a/core/modules/node/templates/node.twig b/core/modules/node/templates/node.html.twig
similarity index 98%
rename from core/modules/node/templates/node.twig
rename to core/modules/node/templates/node.html.twig
index 2d4a674..f1e0b5a 100644
--- a/core/modules/node/templates/node.twig
+++ b/core/modules/node/templates/node.html.twig
@@ -9,7 +9,7 @@
  *   or print a subset such as {{ content.field_example }}. Use
  *   {% hide(content.field_example) %} to temporarily suppress the printing
  *   of a given element.
- * - user_picture: The node author's picture from user-picture.twig.
+ * - user_picture: The node author's picture from user-picture.html.twig.
  * - date: Formatted creation date. Preprocess functions can reformat it by
  *   calling format_date() with the desired parameters on
  *   $variables['created'].
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 6a9d988..13b649c 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
@@ -58,13 +58,13 @@ function testFindThemeTemplates() {
     // Check for correct content
     // @todo Remove this tests once double engine code is removed
 
-    $this->assertEqual($cache['node']['template_file'], 'core/modules/node/templates/node.twig', 'Node is using node.twig as template file');
+    $this->assertEqual($cache['node']['template_file'], 'core/modules/node/templates/node.html.twig', 'Node is using node.html.twig as template file');
     $this->assertEqual($cache['node']['engine'], 'twig', 'Node is using twig engine');
 
     $this->assertEqual($cache['theme_test_template_test']['template_file'], 'core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.tpl.php', 'theme_test is using theme_test.template_test.tpl.php as template file');
     $this->assertEqual($cache['theme_test_template_test']['engine'], 'phptemplate', 'theme_test is using phptemplate as engine.');
 
-    $templates = drupal_find_theme_templates($cache, '.twig', drupal_get_path('theme', 'test_theme_twig'));
-    $this->assertEqual($templates['node__1']['template'], 'node--1', 'Template node--1.twig was found in test_theme_twig.');
+    $templates = drupal_find_theme_templates($cache, '.html.twig', drupal_get_path('theme', 'test_theme_twig'));
+    $this->assertEqual($templates['node__1']['template'], 'node--1', 'Template node--1.html.twig was found in test_theme_twig.');
   }
 }
diff --git a/core/modules/system/templates/datetime.twig b/core/modules/system/templates/datetime.html.twig
similarity index 100%
rename from core/modules/system/templates/datetime.twig
rename to core/modules/system/templates/datetime.html.twig
diff --git a/core/modules/system/tests/themes/test_theme_twig/node--1.html.twig b/core/modules/system/tests/themes/test_theme_twig/node--1.html.twig
new file mode 100644
index 0000000..046c02e
--- /dev/null
+++ b/core/modules/system/tests/themes/test_theme_twig/node--1.html.twig
@@ -0,0 +1,4 @@
+{#
+  // node--1.html.twig - Dummy file for finding the template
+#}
+Node Content Dummy
diff --git a/core/modules/system/tests/themes/test_theme_twig/node--1.twig b/core/modules/system/tests/themes/test_theme_twig/node--1.twig
deleted file mode 100644
index 79148f1..0000000
--- a/core/modules/system/tests/themes/test_theme_twig/node--1.twig
+++ /dev/null
@@ -1,4 +0,0 @@
-{#
-  // node--1.twig - Dummy file for finding the template
-#}
-Node Content Dummy
diff --git a/core/modules/system/tests/themes/test_theme_twig/theme_test.template_test.twig b/core/modules/system/tests/themes/test_theme_twig/theme_test.template_test.html.twig
similarity index 100%
rename from core/modules/system/tests/themes/test_theme_twig/theme_test.template_test.twig
rename to core/modules/system/tests/themes/test_theme_twig/theme_test.template_test.html.twig
diff --git a/core/modules/toolbar/templates/toolbar.twig b/core/modules/toolbar/templates/toolbar.html.twig
similarity index 100%
rename from core/modules/toolbar/templates/toolbar.twig
rename to core/modules/toolbar/templates/toolbar.html.twig
diff --git a/core/themes/engines/twig/twig.engine b/core/themes/engines/twig/twig.engine
index e496bc9..d318263 100644
--- a/core/themes/engines/twig/twig.engine
+++ b/core/themes/engines/twig/twig.engine
@@ -10,14 +10,14 @@
  * Implements hook_theme().
  */
 function twig_theme($existing, $type, $theme, $path) {
-  return drupal_find_theme_templates($existing, '.twig', $path);
+  return drupal_find_theme_templates($existing, '.html.twig', $path);
 }
 
 /**
  * Implements hook_extension().
  */
 function twig_extension() {
-  return '.twig';
+  return '.html.twig';
 }
 
 /**
@@ -45,7 +45,7 @@ function twig_init($template) {
  *   The output generated by the template.
  */
 function twig_render_template($template_file, $variables) {
-  $variables['_references']=array();
+  $variables['_references'] = array();
   return drupal_container()->get('twig')->loadTemplate($template_file)->render($variables);
 }