diff --git a/core/includes/ajax.inc b/core/includes/ajax.inc
index ac1d4cb..1b11f1d 100644
--- a/core/includes/ajax.inc
+++ b/core/includes/ajax.inc
@@ -231,7 +231,7 @@
  *   functions.
  */
 function ajax_render($commands = array()) {
-  // Ajax responses aren't rendered with html.tpl.php, so we have to call
+  // Ajax responses aren't rendered with html.html.twig, so we have to call
   // drupal_get_css() and drupal_get_js() here, in order to have new files added
   // during this request to be loaded by the page. We only want to send back
   // files that the page hasn't already loaded, so we implement simple diffing
diff --git a/core/includes/common.inc b/core/includes/common.inc
index 3c1bdac..8d69595 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -2941,8 +2941,8 @@ function drupal_get_library($module, $name = NULL) {
  * relationships.
  *
  * Note that this function should be called from the theme layer, such as in a
- * .tpl.php file, theme_ function, or in a template_preprocess function, not in
- * a form declaration. Though the same JavaScript could be added to the page
+ * .html.twig file, theme_ function, or in a template_preprocess function, not
+ * in a form declaration. Though the same JavaScript could be added to the page
  * using drupal_add_js() directly, this function helps keep template files
  * clean and readable. It also prevents tabledrag.js from being added twice
  * accidentally.
@@ -2976,7 +2976,6 @@ function drupal_get_library($module, $name = NULL) {
  *   column should not be hidden.
  * @param $limit
  *   (optional) Limit the maximum amount of parenting in this table.
- * @see block-admin-display-form.tpl.php
  * @see theme_menu_overview_form()
  */
 function drupal_add_tabledrag($table_id, $action, $relationship, $group, $subgroup = NULL, $source = NULL, $hidden = TRUE, $limit = 0) {
@@ -3926,7 +3925,7 @@ function drupal_render(&$elements) {
   // children.
   // #states and #attached have to be processed before #theme_wrappers, because
   // the #type 'page' render array from drupal_render_page() would render the
-  // $page and wrap it into the html.tpl.php template without the attached
+  // $page and wrap it into the html.html.twig template without the attached
   // assets otherwise.
   // If the internal #render_children property is set, do not call the
   // #theme_wrappers function(s) to prevent infinite recursion.
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 87b3b18..02a5663 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -815,7 +815,7 @@ function drupal_find_base_themes($themes, $key, $used_keys = array()) {
  * (see hook_theme() for details). Default templates are implemented with the
  * Twig rendering engine and are named the same as the theme hook, with
  * underscores changed to hyphens, so for the 'taxonomy_term' theme hook, the
- * default template is 'taxonomy-term.tpl.php'.
+ * default template is 'taxonomy-term.html.twig'.
  *
  * @subsection sub_overriding_theme_hooks Overriding Theme Hooks
  * Themes may also register new theme hooks within a hook_theme()
@@ -2587,7 +2587,7 @@ function template_preprocess_html(&$variables) {
   $variables['head_title_array'] = $head_title;
   $variables['head_title'] = implode(' | ', $head_title);
 
-  // Display the html.tpl.php's default mobile metatags for responsive design.
+  // Display the html.html.twig's default mobile metatags for responsive design.
   $elements = array(
     'MobileOptimized' => array(
       '#tag' => 'meta',
@@ -2989,13 +2989,17 @@ function template_preprocess_install_page(&$variables) {
 }
 
 /**
- * Preprocess variables for region.tpl.php
+ * Prepares variables for region templates.
+ *
+ * Default template: region.html.twig.
  *
  * Prepares the values passed to the theme_region function to be passed into a
  * pluggable template engine. Uses the region name to generate a template file
- * suggestions. If none are found, the default region.tpl.php is used.
+ * suggestions.
  *
- * @see region.tpl.php
+ * @param array $variables
+ *   An associative array containing:
+ *   - elements: An associative array containing properties of the region.
  */
 function template_preprocess_region(&$variables) {
   // Create the $content variable that templates expect.
diff --git a/core/lib/Drupal/Core/Ajax/AjaxResponse.php b/core/lib/Drupal/Core/Ajax/AjaxResponse.php
index 7660f77..3ab48d0 100644
--- a/core/lib/Drupal/Core/Ajax/AjaxResponse.php
+++ b/core/lib/Drupal/Core/Ajax/AjaxResponse.php
@@ -69,7 +69,7 @@ public function prepare(Request $request) {
    *   An array of commands ready to be returned as JSON.
    */
   protected function ajaxRender(Request $request) {
-    // Ajax responses aren't rendered with html.tpl.php, so we have to call
+    // Ajax responses aren't rendered with html.html.twig, so we have to call
     // drupal_get_css() and drupal_get_js() here, in order to have new files
     // added during this request to be loaded by the page. We only want to send
     // back files that the page hasn't already loaded, so we implement simple
diff --git a/core/modules/aggregator/aggregator.module b/core/modules/aggregator/aggregator.module
index aaffcdc..e5605b8 100644
--- a/core/modules/aggregator/aggregator.module
+++ b/core/modules/aggregator/aggregator.module
@@ -477,7 +477,7 @@ function aggregator_filter_xss($value) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for block.html.twig.
+ * Implements hook_preprocess_HOOK() for block templates.
  */
 function aggregator_preprocess_block(&$variables) {
   if ($variables['configuration']['module'] == 'aggregator') {
diff --git a/core/modules/book/book.module b/core/modules/book/book.module
index 74b1986..90e724b 100644
--- a/core/modules/book/book.module
+++ b/core/modules/book/book.module
@@ -945,7 +945,7 @@ function _book_link_defaults($nid) {
 }
 
  /**
- * Implements hook_preprocess_HOOK() for block.html.twig.
+ * Implements hook_preprocess_HOOK() for block templates.
  */
 function book_preprocess_block(&$variables) {
   if ($variables['configuration']['module'] == 'book') {
diff --git a/core/modules/comment/comment.module b/core/modules/comment/comment.module
index 973bccb..c7ddcfb 100644
--- a/core/modules/comment/comment.module
+++ b/core/modules/comment/comment.module
@@ -1458,7 +1458,7 @@ function comment_preview(Comment $comment) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for block.html.twig.
+ * Implements hook_preprocess_HOOK() for block templates.
  */
 function comment_preprocess_block(&$variables) {
   if ($variables['configuration']['module'] == 'comment') {
diff --git a/core/modules/edit/edit.module b/core/modules/edit/edit.module
index ae02deb..412f5b9 100644
--- a/core/modules/edit/edit.module
+++ b/core/modules/edit/edit.module
@@ -161,7 +161,7 @@ function edit_field_formatter_info_alter(&$info) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for field.tpl.php.
+ * Implements hook_preprocess_HOOK() for field templates.
  */
 function edit_preprocess_field(&$variables) {
   $element = $variables['element'];
diff --git a/core/modules/help/help.module b/core/modules/help/help.module
index 9c86f45..69d4a8b 100644
--- a/core/modules/help/help.module
+++ b/core/modules/help/help.module
@@ -58,7 +58,7 @@ function help_help($path, $arg) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for block.html.twig.
+ * Implements hook_preprocess_HOOK() for block templates.
  */
 function help_preprocess_block(&$variables) {
   if ($variables['plugin_id'] == 'system_help_block') {
diff --git a/core/modules/language/language.module b/core/modules/language/language.module
index 2dd5e57..71ed491 100644
--- a/core/modules/language/language.module
+++ b/core/modules/language/language.module
@@ -775,7 +775,7 @@ function language_language_delete($language) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for block.html.twig.
+ * Implements hook_preprocess_HOOK() for block templates.
  */
 function language_preprocess_block(&$variables) {
   if ($variables['configuration']['module'] == 'language') {
diff --git a/core/modules/locale/locale.module b/core/modules/locale/locale.module
index f0f70ce..6ff3903 100644
--- a/core/modules/locale/locale.module
+++ b/core/modules/locale/locale.module
@@ -883,7 +883,7 @@ function locale_system_file_system_settings_submit(&$form, $form_state) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for node.html.twig.
+ * Implements hook_preprocess_HOOK() for node templates.
  */
 function locale_preprocess_node(&$variables) {
   if ($variables['node']->language()->id != Language::LANGCODE_NOT_SPECIFIED) {
diff --git a/core/modules/menu/menu.module b/core/modules/menu/menu.module
index f564f33..8a14295 100644
--- a/core/modules/menu/menu.module
+++ b/core/modules/menu/menu.module
@@ -697,7 +697,7 @@ function menu_get_menus($all = TRUE) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for block.html.twig.
+ * Implements hook_preprocess_HOOK() for block templates.
  */
 function menu_preprocess_block(&$variables) {
   if ($variables['configuration']['module'] == 'menu') {
diff --git a/core/modules/node/node.api.php b/core/modules/node/node.api.php
index 59d27b5..287cc60 100644
--- a/core/modules/node/node.api.php
+++ b/core/modules/node/node.api.php
@@ -627,7 +627,7 @@ function hook_node_prepare_form(\Drupal\node\NodeInterface $node, $form_display,
  *   theming.
  *
  * @see template_preprocess_search_result()
- * @see search-result.tpl.php
+ * @see search-result.html.twig
  *
  * @ingroup node_api_hooks
  */
diff --git a/core/modules/node/node.module b/core/modules/node/node.module
index 6a86317..7b37166 100644
--- a/core/modules/node/node.module
+++ b/core/modules/node/node.module
@@ -607,7 +607,7 @@ function node_is_page(EntityInterface $node) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for block.html.twig.
+ * Implements hook_preprocess_HOOK() for block templates.
  */
 function node_preprocess_block(&$variables) {
   if ($variables['configuration']['module'] == 'node') {
diff --git a/core/modules/overlay/overlay.module b/core/modules/overlay/overlay.module
index 3788535..b821e45 100644
--- a/core/modules/overlay/overlay.module
+++ b/core/modules/overlay/overlay.module
@@ -327,7 +327,7 @@ function overlay_system_info_alter(&$info, $file, $type) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for html.tpl.php.
+ * Implements hook_preprocess_HOOK() for HTML document templates.
  *
  * If the current page request is inside the overlay, add appropriate classes
  * to the <body> element, and simplify the page title.
@@ -342,7 +342,7 @@ function overlay_preprocess_html(&$variables) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for maintenance-page.html.twig.
+ * Implements hook_preprocess_HOOK() for maintenance page templates.
  *
  * If the current page request is inside the overlay, add appropriate classes
  * to the <body> element, and simplify the page title.
@@ -352,12 +352,16 @@ function overlay_preprocess_maintenance_page(&$variables) {
 }
 
 /**
- * Implements template_preprocess_HOOK() for overlay.tpl.php
+ * Prepares variables for overlay templates.
+ *
+ * Default template: overlay.html.twig.
  *
  * If the current page request is inside the overlay, add appropriate classes
  * to the <body> element, and simplify the page title.
  *
- * @see overlay.tpl.php
+ * @param array $variables
+ *   An associative array containing:
+ *   - page: A render element representing the page.
  */
 function template_preprocess_overlay(&$variables) {
   $variables['tabs'] = menu_primary_local_tasks();
@@ -381,7 +385,7 @@ function template_preprocess_overlay(&$variables) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for page.tpl.php.
+ * Implements hook_preprocess_HOOK() for page templates.
  *
  * If the current page request is inside the overlay, hide the tabs.
  *
diff --git a/core/modules/rdf/rdf.module b/core/modules/rdf/rdf.module
index 428f16b..304c92c 100644
--- a/core/modules/rdf/rdf.module
+++ b/core/modules/rdf/rdf.module
@@ -226,7 +226,7 @@ function rdf_theme() {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for html.tpl.php.
+ * Implements hook_preprocess_HOOK() for HTML document templates.
  */
 function rdf_preprocess_html(&$variables) {
   // Adds RDF namespace prefix bindings in the form of an RDFa 1.1 prefix
@@ -241,7 +241,7 @@ function rdf_preprocess_html(&$variables) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for node.html.twig.
+ * Implements hook_preprocess_HOOK() for node templates.
  */
 function rdf_preprocess_node(&$variables) {
   // Adds RDFa markup to the node container. The about attribute specifies the
@@ -318,7 +318,7 @@ function rdf_preprocess_node(&$variables) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for field.tpl.php.
+ * Implements hook_preprocess_HOOK() for field templates.
  */
 function rdf_preprocess_field(&$variables) {
   $element = $variables['element'];
@@ -355,7 +355,7 @@ function rdf_preprocess_field(&$variables) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for user.tpl.php.
+ * Implements hook_preprocess_HOOK() for user templates.
  */
 function rdf_preprocess_user(&$variables) {
   $account = $variables['elements']['#user'];
@@ -449,7 +449,7 @@ function rdf_preprocess_username(&$variables) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for comment.html.twig.
+ * Implements hook_preprocess_HOOK() for comment templates.
  */
 function rdf_preprocess_comment(&$variables) {
   $comment = $variables['comment'];
@@ -538,7 +538,7 @@ function rdf_preprocess_comment(&$variables) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for taxonomy-term.tpl.php.
+ * Implements hook_preprocess_HOOK() for taxonomy term templates.
  */
 function rdf_preprocess_taxonomy_term(&$variables) {
   $term = $variables['term'];
diff --git a/core/modules/search/search.module b/core/modules/search/search.module
index a4830ad..cab1a5f 100644
--- a/core/modules/search/search.module
+++ b/core/modules/search/search.module
@@ -137,7 +137,7 @@ function search_permission() {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for block.html.twig.
+ * Implements hook_preprocess_HOOK() for block templates.
  */
 function search_preprocess_block(&$variables) {
   if ($variables['plugin_id'] == 'search_form_block') {
diff --git a/core/modules/shortcut/shortcut.module b/core/modules/shortcut/shortcut.module
index 1d1a505..7f30ca0 100644
--- a/core/modules/shortcut/shortcut.module
+++ b/core/modules/shortcut/shortcut.module
@@ -442,7 +442,7 @@ function shortcut_renderable_links($shortcut_set = NULL) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for block.html.twig.
+ * Implements hook_preprocess_HOOK() for block templates.
  */
 function shortcut_preprocess_block(&$variables) {
   if ($variables['configuration']['module'] == 'shortcut') {
@@ -451,7 +451,7 @@ function shortcut_preprocess_block(&$variables) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for page.tpl.php.
+ * Implements hook_preprocess_HOOK() for page templates.
  */
 function shortcut_preprocess_page(&$variables) {
   // Only display the shortcut link if the user has the ability to edit
diff --git a/core/modules/statistics/statistics.module b/core/modules/statistics/statistics.module
index 853d139..7100547 100644
--- a/core/modules/statistics/statistics.module
+++ b/core/modules/statistics/statistics.module
@@ -237,7 +237,7 @@ function statistics_update_index() {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for block.html.twig.
+ * Implements hook_preprocess_HOOK() for block templates.
  */
 function statistics_preprocess_block(&$variables) {
   if ($variables['configuration']['module'] == 'statistics') {
diff --git a/core/modules/system/lib/Drupal/system/Tests/FileTransfer/FileTransferTest.php b/core/modules/system/lib/Drupal/system/Tests/FileTransfer/FileTransferTest.php
index 066f854..a08a42e 100644
--- a/core/modules/system/lib/Drupal/system/Tests/FileTransfer/FileTransferTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/FileTransfer/FileTransferTest.php
@@ -38,7 +38,7 @@ function _getFakeModuleFiles() {
       'fake.module',
       'fake.info.yml',
       'theme' => array(
-        'fake.tpl.php'
+        'fake.html.twig'
       ),
       'inc' => array(
         'fake.inc'
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/HtmlAttributesTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/HtmlAttributesTest.php
new file mode 100644
index 0000000..2ba1ebc
--- /dev/null
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/HtmlAttributesTest.php
@@ -0,0 +1,42 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\Tests\Theme\HtmlAttributesTest.
+ */
+
+namespace Drupal\system\Tests\Theme;
+
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Functional tests for 'html' and 'body' element attributes.
+ */
+class HtmlAttributesTest extends WebTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = array('theme_test');
+
+  public static function getInfo() {
+    return array(
+      'name' => "'html' and 'body' element attributes",
+      'description' => "Test attributes inserted in the 'html' and 'body' elements on the page.",
+      'group' => 'Theme',
+    );
+  }
+
+  /**
+   * Tests that attributes in the 'html' and 'body' elements can be altered.
+   */
+  function testThemeHtmlAttributes() {
+    $this->drupalGet('');
+    $attributes = $this->xpath('/html[@theme_test_html_attribute="theme test html attribute value"]');
+    $this->assertTrue(count($attributes) == 1, "Attribute set in the 'html' element via hook_preprocess_HOOK() found.");
+    $attributes = $this->xpath('/html/body[@theme_test_body_attribute="theme test body attribute value"]');
+    $this->assertTrue(count($attributes) == 1, "Attribute set in the 'body' element via hook_preprocess_HOOK() found.");
+  }
+}
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/HtmlTplPhpAttributesTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/HtmlTplPhpAttributesTest.php
deleted file mode 100644
index d06b165..0000000
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/HtmlTplPhpAttributesTest.php
+++ /dev/null
@@ -1,42 +0,0 @@
-<?php
-
-/**
- * @file
- * Definition of Drupal\system\Tests\Theme\HtmlTplPhpAttributesTest.
- */
-
-namespace Drupal\system\Tests\Theme;
-
-use Drupal\simpletest\WebTestBase;
-
-/**
- * Functional test for attributes of html.tpl.php.
- */
-class HtmlTplPhpAttributesTest extends WebTestBase {
-
-  /**
-   * Modules to enable.
-   *
-   * @var array
-   */
-  public static $modules = array('theme_test');
-
-  public static function getInfo() {
-    return array(
-      'name' => 'html.tpl.php html and body attributes',
-      'description' => 'Tests attributes inserted in the html and body elements of html.tpl.php.',
-      'group' => 'Theme',
-    );
-  }
-
-  /**
-   * Tests that modules and themes can alter variables in html.tpl.php.
-   */
-  function testThemeHtmlTplPhpAttributes() {
-    $this->drupalGet('');
-    $attributes = $this->xpath('/html[@theme_test_html_attribute="theme test html attribute value"]');
-    $this->assertTrue(count($attributes) == 1, 'Attribute set in the html element via hook_preprocess_HOOK() for html.tpl.php found.');
-    $attributes = $this->xpath('/html/body[@theme_test_body_attribute="theme test body attribute value"]');
-    $this->assertTrue(count($attributes) == 1, 'Attribute set in the body element via hook_preprocess_HOOK() for html.tpl.php found.');
-  }
-}
diff --git a/core/modules/system/system.api.php b/core/modules/system/system.api.php
index d366e01..2e2abf9 100644
--- a/core/modules/system/system.api.php
+++ b/core/modules/system/system.api.php
@@ -1553,7 +1553,7 @@ function hook_permission() {
  *     path, include it here. This path should be relative to the Drupal root
  *     directory.
  *   - template: If specified, this theme implementation is a template, and
- *     this is the template file without an extension. Do not put .tpl.php on
+ *     this is the template file without an extension. Do not put .html.twig on
  *     this file; that extension will be added automatically by the default
  *     rendering engine (which is Twig). If 'path' above is specified, the
  *     template should also be in this path.
@@ -1583,7 +1583,7 @@ function hook_permission() {
  *   - override preprocess functions: Set to TRUE when a theme does NOT want
  *     the standard preprocess functions to run. This can be used to give a
  *     theme FULL control over how variables are set. For example, if a theme
- *     wants total control over how certain variables in the page.tpl.php are
+ *     wants total control over how certain variables in the page.html.twig are
  *     set, this can be set to true. Please keep in mind that when this is used
  *     by a theme, that theme becomes responsible for making sure necessary
  *     variables are set.
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index dfda891..f624399 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -2279,7 +2279,7 @@ function system_user_timezone(&$form, &$form_state) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for block.html.twig.
+ * Implements hook_preprocess_HOOK() for block templates.
  */
 function system_preprocess_block(&$variables) {
   // Derive the base plugin ID.
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 40a78f3..8cbcde8 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -150,7 +150,7 @@ function _theme_test_suggestion() {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for html.tpl.php.
+ * Implements hook_preprocess_HOOK() for HTML document templates.
  */
 function theme_test_preprocess_html(&$variables) {
   $variables['html_attributes']['theme_test_html_attribute'] = 'theme test html attribute value';
@@ -184,7 +184,13 @@ function theme_theme_test_function_template_override($variables) {
 }
 
 /**
- * Process variables for theme-test-render-element.tpl.php.
+ * Prepares variables for test render element templates.
+ *
+ * Default template: theme-test-render-element.html.twig.
+ *
+ * @param array $variables
+ *   An associative array containing:
+ *   - elements: An associative array containing the properties of the element.
  */
 function template_preprocess_theme_test_render_element(&$variables) {
   $variables['attributes']['data-variables-are-preprocessed'] = TRUE;
diff --git a/core/modules/taxonomy/taxonomy.api.php b/core/modules/taxonomy/taxonomy.api.php
index 17ec07f..d174a4a 100644
--- a/core/modules/taxonomy/taxonomy.api.php
+++ b/core/modules/taxonomy/taxonomy.api.php
@@ -281,7 +281,7 @@ function hook_taxonomy_term_view(\Drupal\taxonomy\Entity\Term $term, \Drupal\ent
  * If the module wishes to act on the rendered HTML of the term rather than the
  * structured content array, it may use this hook to add a #post_render
  * callback. Alternatively, it could also implement
- * hook_preprocess_HOOK() for taxonomy-term.tpl.php. See drupal_render() and
+ * hook_preprocess_HOOK() for taxonomy-term.html.twig. See drupal_render() and
  * theme() documentation respectively for details.
  *
  * @param $build
diff --git a/core/modules/tour/tour.module b/core/modules/tour/tour.module
index 9cb4f64..0945ad7 100644
--- a/core/modules/tour/tour.module
+++ b/core/modules/tour/tour.module
@@ -104,7 +104,7 @@ function tour_toolbar() {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for page.tpl.php.
+ * Implements hook_preprocess_HOOK() for page templates.
  */
 function tour_preprocess_page(&$variables) {
   if (!Drupal::currentUser()->hasPermission('access tour')) {
diff --git a/core/modules/user/user.api.php b/core/modules/user/user.api.php
index b7f8b8c..dda1fa0 100644
--- a/core/modules/user/user.api.php
+++ b/core/modules/user/user.api.php
@@ -346,7 +346,7 @@ function hook_user_view(\Drupal\user\UserInterface $account, \Drupal\entity\Enti
  * If the module wishes to act on the rendered HTML of the user rather than the
  * structured content array, it may use this hook to add a #post_render callback.
  * Alternatively, it could also implement hook_preprocess_HOOK() for
- * user.tpl.php. See drupal_render() and theme() documentation
+ * user.html.twig. See drupal_render() and theme() documentation
  * respectively for details.
  *
  * @param $build
diff --git a/core/modules/user/user.module b/core/modules/user/user.module
index 1f50de0..fcb1bdd 100644
--- a/core/modules/user/user.module
+++ b/core/modules/user/user.module
@@ -1338,7 +1338,7 @@ function user_view_page($account) {
  * - $page['content']['#user']:
  *   The user account of the profile being viewed.
  *
- * To theme user profiles, copy modules/user/user.tpl.php
+ * To theme user profiles, copy core/modules/user/templates/user.html.twig
  * to your theme directory, and edit it as instructed in that file's comments.
  *
  * @param $account
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/PluginBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/PluginBase.php
index a291143..817488e 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/PluginBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/PluginBase.php
@@ -25,7 +25,7 @@
  *   If a template file should be used, the file has to be placed in the
  *   module's templates folder.
  *   Example: theme = "mymodule_row" of module "mymodule" will implement either
- *   theme_mymodule_row() or mymodule-row.tpl.php in the
+ *   theme_mymodule_row() or mymodule-row.html.twig in the
  *   [..]/modules/mymodule/templates folder.
  * - register_theme: (optional) When set to TRUE (default) the theme is
  *   registered automatically. When set to FALSE the plugin reuses an existing
diff --git a/core/modules/views/tests/modules/views_test_data/views_test_data.module b/core/modules/views/tests/modules/views_test_data/views_test_data.module
index e5dc45e..75f0613 100644
--- a/core/modules/views/tests/modules/views_test_data/views_test_data.module
+++ b/core/modules/views/tests/modules/views_test_data/views_test_data.module
@@ -56,7 +56,7 @@ function views_test_data_handler_test_access_callback_argument($argument = FALSE
 }
 
 /**
- * Implements hook_preprocess_HOOK() for views-view-table.tpl.php.
+ * Implements hook_preprocess_HOOK() for views table templates.
  */
 function views_test_data_preprocess_views_view_table(&$variables) {
   if ($variables['view']->storage->id() == 'test_view_render') {
diff --git a/core/modules/views/views.module b/core/modules/views/views.module
index e46f7fe..94199c2 100644
--- a/core/modules/views/views.module
+++ b/core/modules/views/views.module
@@ -496,8 +496,8 @@ function views_preprocess_html(&$variables) {
   // associated with. This we don't want; for better visual highlighting, we
   // prefer a smaller region to be chosen. The region we prefer differs from
   // theme to theme and depends on the details of the theme's markup in
-  // page.tpl.php, so we can only find it using JavaScript. We therefore remove
-  // the "contextual-region" class from the <body> tag here and add
+  // page.html.twig, so we can only find it using JavaScript. We therefore
+  // remove the "contextual-region" class from the <body> tag here and add
   // JavaScript that will insert it back in the correct place.
   if (!empty($variables['page']['#views_contextual_links'])) {
     $key = array_search('contextual-region', $variables['attributes']['class']);
diff --git a/core/modules/views/views.theme.inc b/core/modules/views/views.theme.inc
index 2594138..5213679 100644
--- a/core/modules/views/views.theme.inc
+++ b/core/modules/views/views.theme.inc
@@ -1183,16 +1183,16 @@ function template_preprocess_views_mini_pager(&$variables) {
  * The default style for all views is views-view-unformatted.html.twig.
  *
  * Many styles will then farm out the actual display of each row to a row
- * style; the default row style is views-view-fields.tpl.php.
+ * style; the default row style is views-view-fields.html.twig.
  *
  * Here is an example of all the templates that will be tried in the following
  * case:
  *
  * View, named foobar. Style: unformatted. Row style: Fields. Display: Page.
  *
- * - views-view--foobar--page.tpl.php
- * - views-view--page.tpl.php
- * - views-view--foobar.tpl.php
+ * - views-view--foobar--page.html.twig
+ * - views-view--page.html.twig
+ * - views-view--foobar.html.twig
  * - views-view.html.twig
  *
  * - views-view-unformatted--foobar--page.html.twig
@@ -1200,10 +1200,10 @@ function template_preprocess_views_mini_pager(&$variables) {
  * - views-view-unformatted--foobar.html.twig
  * - views-view-unformatted.html.twig
  *
- * - views-view-fields--foobar--page.tpl.php
- * - views-view-fields--page.tpl.php
- * - views-view-fields--foobar.tpl.php
- * - views-view-fields.tpl.php
+ * - views-view-fields--foobar--page.html.twig
+ * - views-view-fields--page.html.twig
+ * - views-view-fields--foobar.html.twig
+ * - views-view-fields.html.twig
  *
  * Important! When adding a new template to your theme, be sure to flush the
  * theme registry cache!
diff --git a/core/modules/views_ui/js/views-admin.js b/core/modules/views_ui/js/views-admin.js
index d20b0af..7acff80 100644
--- a/core/modules/views_ui/js/views-admin.js
+++ b/core/modules/views_ui/js/views-admin.js
@@ -488,9 +488,9 @@ $.extend(Drupal.viewsUi.RearrangeFilterHandler.prototype, {
   insertAddRemoveFilterGroupLinks: function () {
 
     // Insert a link for adding a new group at the top of the page, and make it
-    // match the action links styling used in a typical page.tpl.php. Note that
-    // Drupal does not provide a theme function for this markup, so this is the
-    // best we can do.
+    // match the action link styling used in a typical page.html.twig. Since
+    // Drupal does not provide a theme function for this markup this is the best
+    // we can do.
     $('<ul class="action-links"><li><a id="views-add-group-link" href="#">' + this.addGroupButton.val() + '</a></li></ul>')
       .prependTo(this.table.parent())
       // When the link is clicked, dynamically click the hidden form button for
diff --git a/core/modules/views_ui/views_ui.module b/core/modules/views_ui/views_ui.module
index 9e61cfd..4098320 100644
--- a/core/modules/views_ui/views_ui.module
+++ b/core/modules/views_ui/views_ui.module
@@ -214,7 +214,7 @@ function views_ui_library_info() {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for views-view.html.twig.
+ * Implements hook_preprocess_HOOK() for views templates.
  */
 function views_ui_preprocess_views_view(&$variables) {
   $view = $variables['view'];
diff --git a/core/themes/bartik/bartik.theme b/core/themes/bartik/bartik.theme
index ed59e8d..02f0faf 100644
--- a/core/themes/bartik/bartik.theme
+++ b/core/themes/bartik/bartik.theme
@@ -6,7 +6,7 @@
  */
 
 /**
- * Implements hook_preprocess_HOOK() for html.tpl.php.
+ * Implements hook_preprocess_HOOK() for HTML document templates.
  *
  * Adds body classes if certain regions have content.
  */
@@ -30,7 +30,7 @@ function bartik_preprocess_html(&$variables) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for page.html.twig.
+ * Implements hook_preprocess_HOOK() for page templates.
  */
 function bartik_preprocess_page(&$variables) {
   // Pass the main menu and secondary menu to the template as render arrays.
@@ -74,7 +74,7 @@ function bartik_preprocess_page(&$variables) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for maintenance-page.html.twig.
+ * Implements hook_preprocess_HOOK() for maintenance page templates.
  */
 function bartik_preprocess_maintenance_page(&$variables) {
   // By default, site_name is set to Drupal if no db connection is available
@@ -103,7 +103,7 @@ function bartik_preprocess_maintenance_page(&$variables) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for node.html.twig.
+ * Implements hook_preprocess_HOOK() for node templates.
  */
 function bartik_preprocess_node(&$variables) {
   // Remove the "Add new comment" link on teasers or when the comment form is
diff --git a/core/themes/seven/seven.theme b/core/themes/seven/seven.theme
index ceb4ab5..49cce25 100644
--- a/core/themes/seven/seven.theme
+++ b/core/themes/seven/seven.theme
@@ -34,8 +34,8 @@ function seven_library_info() {
  * Implements hook_preprocess_HOOK() for maintenance-page.html.twig.
  */
 function seven_preprocess_maintenance_page(&$variables) {
-  // While markup for normal pages is split into page.tpl.php and html.tpl.php,
-  // the markup for the maintenance page is all in the single
+  // While markup for normal pages is split into page.html.twig and
+  // html.html.twig, the markup for the maintenance page is all in the single
   // maintenance-page.html.twig template. So, to have what's done in
   // seven_preprocess_html() also happen on the maintenance page, it has to be
   // called here.
@@ -43,14 +43,14 @@ function seven_preprocess_maintenance_page(&$variables) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for html.tpl.php.
+ * Implements hook_preprocess_HOOK() for HTML document templates.
  */
 function seven_preprocess_html(&$variables) {
   drupal_add_library('system', 'normalize');
 }
 
 /**
- * Implements hook_preprocess_HOOK() for page.tpl.php.
+ * Implements hook_preprocess_HOOK() for page templates.
  */
 function seven_preprocess_page(&$variables) {
   $variables['primary_local_tasks'] = $variables['tabs'];
diff --git a/sites/default/default.settings.php b/sites/default/default.settings.php
index a58d60e..192840a 100644
--- a/sites/default/default.settings.php
+++ b/sites/default/default.settings.php
@@ -578,7 +578,7 @@
  * is explicitly set to maintenance mode through the administration page or when
  * the database is inactive due to an error. It can be set through the
  * 'maintenance_theme' key. The template file should also be copied into the
- * theme. It is located inside 'core/modules/system/maintenance-page.tpl.php'.
+ * theme. It is located inside 'core/modules/system/maintenance-page.html.twig'.
  * Note: This setting does not apply to installation and update pages.
  */
 # $conf['maintenance_theme'] = 'bartik';