diff --git a/core/includes/common.inc b/core/includes/common.inc
index 4668a90..add08eb 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -5913,7 +5913,7 @@ function element_set_attributes(array &$element, array $map) {
  * - name: The real name of the theme for display purposes.
  * - description: Brief description.
  * - screenshot: Path to screenshot relative to the theme's .info.yml file.
- * - engine: Theme engine; typically phptemplate.
+ * - engine: Theme engine; typically twig.
  * - base theme: Name of a base theme, if applicable.
  * - regions: Listed regions.
  * - features: Features available.
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 85ca1fa..e89cb8f 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -267,11 +267,9 @@ function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callb
       include_once DRUPAL_ROOT . '/' . $theme->owner;
     }
   }
-  // Load twig as secondary always available engine.
-  // @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 .html.twig conversion.
+
+  // Always include Twig so that PHPTemplate themes do not have to convert all
+  // core and contrib templates in order to use the PHPTemplate engine.
   include_once DRUPAL_ROOT . '/core/themes/engines/twig/twig.engine';
 
   if (isset($registry_callback)) {
@@ -512,28 +510,6 @@ function _theme_process_registry(&$cache, $name, $type, $theme, $path) {
         if (!isset($info['path'])) {
           $result[$hook]['template'] = $path . '/templates/' . $info['template'];
         }
-        if ($type == 'module' || $type == 'theme_engine') {
-          // Add two render engines for modules and theme engines.
-          // @todo Remove and make twig the default engine.
-          $render_engines = array(
-            '.html.twig' => 'twig',
-            '.tpl.php' => 'phptemplate'
-          );
-
-          // Find the best engine for this template.
-          foreach ($render_engines as $extension => $engine) {
-            // Render the output using the template file.
-            $template_file = $result[$hook]['template'] . $extension;
-            if (isset($info['path'])) {
-              $template_file = $info['path'] . '/' . $template_file;
-            }
-            if (file_exists($template_file)) {
-              $result[$hook]['template_file'] = $template_file;
-              $result[$hook]['engine'] = $engine;
-              break;
-            }
-          }
-        }
       }
 
       // Allow variable processors for all theming hooks, whether the hook is
@@ -665,12 +641,6 @@ function _theme_build_registry($theme, $base_theme, $theme_engine) {
     _theme_process_registry($cache, $theme_engine, 'theme_engine', $theme->name, dirname($theme->filename));
   }
 
-  if ($theme_engine == 'phptemplate') {
-    // Check for Twig templates if this is a PHPTemplate theme.
-    // @todo Remove this once all core themes are converted to Twig.
-    _theme_process_registry($cache, 'twig', 'theme_engine', $theme->name, dirname($theme->filename));
-  }
-
   // Finally, hooks provided by the theme itself.
   _theme_process_registry($cache, $theme->name, 'theme', $theme->name, dirname($theme->filename));
 
@@ -846,7 +816,7 @@ function drupal_find_base_themes($themes, $key, $used_keys = array()) {
  * theme_HOOK() (e.g., theme_taxonomy_term()) or via a template file named
  * according to the value of the 'template' key registered with the theme hook
  * (see hook_theme() for details). Default templates are implemented with the
- * PHPTemplate rendering engine and are named the same as the theme hook, with
+ * Twig rendering engine and are named the same as the theme hook, with
  * underscores changed to hyphens, so for the 'taxonomy_term' theme hook, the
  * default template is 'taxonomy-term.tpl.php'.
  *
@@ -858,9 +828,9 @@ function drupal_find_base_themes($themes, $key, $used_keys = array()) {
  * default implementation of the 'menu_tree' theme hook by implementing a
  * bartik_menu_tree() function), or by adding a template file within its folder
  * structure that follows the template naming structure used by the theme's
- * rendering engine (for example, since the Bartik theme uses the PHPTemplate
- * rendering engine, it overrides the default implementation of the 'page' theme
- * hook by containing a 'page.tpl.php' file within its folder structure).
+ * rendering engine (for example, since the Bartik theme uses the Twig rendering
+ * engine, it overrides the default implementation of the 'page' theme hook by
+ * containing a 'page.html.twig' file within its folder structure).
  *
  * If the implementation is a template file, several functions are called
  * before the template file is invoked, to modify the $variables array. These
@@ -1120,9 +1090,8 @@ function theme($hook, $variables = array()) {
     }
   }
   else {
-    // Default render function and extension.
-    $render_function = 'theme_render_template';
-    $extension = '.tpl.php';
+    $render_function = 'twig_render_template';
+    $extension = '.html.twig';
 
     // The theme engine may use a different extension and a different renderer.
     global $theme_engine;
@@ -1136,13 +1105,6 @@ function theme($hook, $variables = array()) {
           $extension = $extension_function();
         }
       }
-      // @todo Change this 'if' back to 'elseif' once Twig conversion is
-      // complete. Normally only modules have this key set.
-      if (isset($info['engine'])) {
-        if (function_exists($info['engine'] . '_render_template')) {
-          $render_function = $info['engine'] . '_render_template';
-        }
-      }
     }
 
     // In some cases, a template implementation may not have had
@@ -1180,12 +1142,6 @@ function theme($hook, $variables = array()) {
     if (isset($info['path'])) {
       $template_file = $info['path'] . '/' . $template_file;
     }
-
-    // Modules can override this.
-    if (isset($info['template_file'])) {
-      $template_file = $info['template_file'];
-    }
-
     $output = $render_function($template_file, $variables);
   }
 
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
index 20ccc2b..f30491e 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -154,10 +154,10 @@ function testListThemes() {
     $this->assertIdentical($themes['test_basetheme']->sub_themes, $sub_theme_list, 'Base theme\'s object includes list of subthemes.');
     $this->assertIdentical($themes['test_subtheme']->base_themes, $base_theme_list, 'Subtheme\'s object includes list of base themes.');
     // Check for theme engine in subtheme.
-    $this->assertIdentical($themes['test_subtheme']->engine, 'phptemplate', 'Subtheme\'s object includes the theme engine.');
+    $this->assertIdentical($themes['test_subtheme']->engine, 'twig', 'Subtheme\'s object includes the theme engine.');
     // Check for theme engine prefix.
-    $this->assertIdentical($themes['test_basetheme']->prefix, 'phptemplate', 'Base theme\'s object includes the theme engine prefix.');
-    $this->assertIdentical($themes['test_subtheme']->prefix, 'phptemplate', 'Subtheme\'s object includes the theme engine prefix.');
+    $this->assertIdentical($themes['test_basetheme']->prefix, 'twig', 'Base theme\'s object includes the theme engine prefix.');
+    $this->assertIdentical($themes['test_subtheme']->prefix, 'twig', 'Subtheme\'s object includes the theme engine prefix.');
   }
 
   /**
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 5ac99e0..135a435 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
@@ -31,7 +31,7 @@ public static function getInfo() {
 
   function setUp() {
     parent::setUp();
-    theme_enable(array('test_theme_twig'));
+    theme_enable(array('test_theme'));
   }
 
   /**
@@ -39,43 +39,18 @@ function setUp() {
    */
   function testTemplateOverride() {
     config('system.theme')
-      ->set('default', 'test_theme_twig')
+      ->set('default', 'test_theme')
       ->save();
     $this->drupalGet('theme-test/template-test');
     $this->assertText('Success: Template overridden.', t('Template overridden by defined \'template\' filename.'));
   }
 
   /**
-   * Tests drupal_find_theme_templates
-   */
-  function testFindThemeTemplates() {
-
-    $cache = array();
-
-    // Prime the theme cache
-    foreach (module_implements('theme') as $module) {
-      _theme_process_registry($cache, $module, 'module', $module, drupal_get_path('module', $module));
-    }
-
-    // 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.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, '.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.');
-  }
-
-  /**
    * Tests that the Twig engine handles PHP data correctly.
    */
   function testTwigVariableDataTypes() {
     config('system.theme')
-      ->set('default', 'test_theme_twig')
+      ->set('default', 'test_theme')
       ->save();
     $this->drupalGet('twig-theme-test/php-variables');
     foreach (_test_theme_twig_php_values() as $type => $value) {
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
index 0bcd1a6..38e183b 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
@@ -34,8 +34,8 @@ public static function getInfo() {
    */
   function testTwigDebugMarkup() {
     $extension = twig_extension();
-    theme_enable(array('test_theme_twig'));
-    config('system.theme')->set('default', 'test_theme_twig')->save();
+    theme_enable(array('test_theme'));
+    config('system.theme')->set('default', 'test_theme')->save();
     // Enable debug, rebuild the service container, and clear all caches.
     $this->settingsSet('twig_debug', TRUE);
     $this->rebuildContainer();
@@ -47,7 +47,7 @@ function testTwigDebugMarkup() {
       _theme_process_registry($cache, $module, 'module', $module, drupal_get_path('module', $module));
     }
     // Create array of Twig templates.
-    $templates = drupal_find_theme_templates($cache, $extension, drupal_get_path('theme', 'test_theme_twig'));
+    $templates = drupal_find_theme_templates($cache, $extension, drupal_get_path('theme', 'test_theme'));
     $templates += drupal_find_theme_templates($cache, $extension, drupal_get_path('module', 'node'));
 
     // Create a node and test different features of the debug markup.
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigSettingsTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigSettingsTest.php
index 2e84924..f50f110 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigSettingsTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigSettingsTest.php
@@ -77,9 +77,9 @@ function testTwigDebugOverride() {
    */
   function testTwigCacheOverride() {
     $extension = twig_extension();
-    theme_enable(array('test_theme_twig'));
+    theme_enable(array('test_theme'));
     config('system.theme')
-      ->set('default', 'test_theme_twig')
+      ->set('default', 'test_theme')
       ->save();
 
     $cache = array();
@@ -89,7 +89,7 @@ function testTwigCacheOverride() {
     }
 
     // Load array of Twig templates.
-    $templates = drupal_find_theme_templates($cache, $extension, drupal_get_path('theme', 'test_theme_twig'));
+    $templates = drupal_find_theme_templates($cache, $extension, drupal_get_path('theme', 'test_theme'));
 
     // Get the template filename and the cache filename for
     // theme_test.template_test.html.twig.
diff --git a/core/modules/system/system.api.php b/core/modules/system/system.api.php
index 5360a83..1f0c4a9 100644
--- a/core/modules/system/system.api.php
+++ b/core/modules/system/system.api.php
@@ -1589,8 +1589,8 @@ function hook_permission() {
  *   - 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 file; that extension will be added automatically by the default
- *     rendering engine (which is PHPTemplate). If 'path', above, is specified,
- *     the template should also be in this path.
+ *     rendering engine (which is Twig). If 'path' above is specified, the
+ *     template should also be in this path.
  *   - function: If specified, this will be the function name to invoke for
  *     this implementation. If neither 'template' nor 'function' is specified,
  *     a default function name will be assumed. For example, if a module
@@ -1611,8 +1611,8 @@ function hook_permission() {
  *   - preprocess functions: A list of functions used to preprocess this data.
  *     Ordinarily this won't be used; it's automatically filled in. By default,
  *     for a module this will be filled in as template_preprocess_HOOK. For
- *     a theme this will be filled in as phptemplate_preprocess and
- *     phptemplate_preprocess_HOOK as well as themename_preprocess and
+ *     a theme this will be filled in as twig_preprocess and
+ *     twig_preprocess_HOOK as well as themename_preprocess and
  *     themename_preprocess_HOOK.
  *   - 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
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 4d2f61e..edad0eb 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -3011,7 +3011,7 @@ function _system_rebuild_theme_data() {
 
   // Set defaults for theme info.
   $defaults = array(
-    'engine' => 'phptemplate',
+    'engine' => 'twig',
     'regions' => array(
       'sidebar_first' => 'Left sidebar',
       'sidebar_second' => 'Right sidebar',
diff --git a/core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.tpl.php b/core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.html.twig
similarity index 100%
rename from core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.tpl.php
rename to core/modules/system/tests/modules/theme_test/templates/theme_test.template_test.html.twig
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 137fdfc..e72334c 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -27,7 +27,7 @@ function theme_test_system_theme_info() {
   $themes['test_theme'] = drupal_get_path('module', 'system') . '/tests/themes/test_theme/test_theme.info.yml';
   $themes['test_basetheme'] = drupal_get_path('module', 'system') . '/tests/themes/test_basetheme/test_basetheme.info.yml';
   $themes['test_subtheme'] = drupal_get_path('module', 'system') . '/tests/themes/test_subtheme/test_subtheme.info.yml';
-  $themes['test_theme_twig'] = drupal_get_path('module', 'system') . '/tests/themes/test_theme_twig/test_theme_twig.info.yml';
+  $themes['test_theme_phptemplate'] = drupal_get_path('module', 'system') . '/tests/themes/test_theme_phptemplate/test_theme_phptemplate.info.yml';
   return $themes;
 }
 
diff --git a/core/modules/system/tests/themes/test_theme_twig/node--1.html.twig b/core/modules/system/tests/themes/test_theme/node--1.html.twig
similarity index 100%
rename from core/modules/system/tests/themes/test_theme_twig/node--1.html.twig
rename to core/modules/system/tests/themes/test_theme/node--1.html.twig
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 8275818..62b5abf 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.theme
+++ b/core/modules/system/tests/themes/test_theme/test_theme.theme
@@ -1,6 +1,18 @@
 <?php
 
 /**
+ * @file
+ * Theme to help test the Twig engine.
+ */
+
+/**
+ * Implements THEME_preprocess_twig_theme_test_php_variables().
+ */
+function test_theme_preprocess_twig_theme_test_php_variables(&$variables) {
+  $variables['php_values'] = _test_theme_twig_php_values();
+}
+
+/**
  * Tests a theme overriding a suggestion of a base theme hook.
  */
 function test_theme_theme_test__suggestion($variables) {
diff --git a/core/modules/system/tests/themes/test_theme_twig/theme_test.template_test.html.twig b/core/modules/system/tests/themes/test_theme/theme_test.template_test.html.twig
similarity index 100%
rename from core/modules/system/tests/themes/test_theme_twig/theme_test.template_test.html.twig
rename to core/modules/system/tests/themes/test_theme/theme_test.template_test.html.twig
diff --git a/core/modules/system/tests/themes/test_theme/node--1.tpl.php b/core/modules/system/tests/themes/test_theme_phptemplate/node--1.tpl.php
similarity index 100%
rename from core/modules/system/tests/themes/test_theme/node--1.tpl.php
rename to core/modules/system/tests/themes/test_theme_phptemplate/node--1.tpl.php
diff --git a/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.info.yml b/core/modules/system/tests/themes/test_theme_phptemplate/test_theme_phptemplate.info.yml
similarity index 21%
rename from core/modules/system/tests/themes/test_theme_twig/test_theme_twig.info.yml
rename to core/modules/system/tests/themes/test_theme_phptemplate/test_theme_phptemplate.info.yml
index 2c4cf83..fa975b9 100644
--- a/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.info.yml
+++ b/core/modules/system/tests/themes/test_theme_phptemplate/test_theme_phptemplate.info.yml
@@ -1,6 +1,6 @@
-name: 'Test theme for Twig'
+name: 'Test theme PHPTemplate'
 type: theme
-description: 'Theme for testing the theme system with twig engine'
-engine: twig
+description: 'Theme for testing the theme system with the PHPTemplate engine'
 core: 8.x
+engine: phptemplate
 hidden: true
diff --git a/core/modules/system/tests/themes/test_theme_twig/template.php b/core/modules/system/tests/themes/test_theme_phptemplate/test_theme_phptemplate.theme
similarity index 85%
rename from core/modules/system/tests/themes/test_theme_twig/template.php
rename to core/modules/system/tests/themes/test_theme_phptemplate/test_theme_phptemplate.theme
index a4abe2d..b3d9bbc 100644
--- a/core/modules/system/tests/themes/test_theme_twig/template.php
+++ b/core/modules/system/tests/themes/test_theme_phptemplate/test_theme_phptemplate.theme
@@ -1,2 +1 @@
 <?php
-
diff --git a/core/modules/system/tests/themes/test_theme/theme_test.template_test.tpl.php b/core/modules/system/tests/themes/test_theme_phptemplate/theme_test.template_test.tpl.php
similarity index 100%
rename from core/modules/system/tests/themes/test_theme/theme_test.template_test.tpl.php
rename to core/modules/system/tests/themes/test_theme_phptemplate/theme_test.template_test.tpl.php
diff --git a/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme b/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme
deleted file mode 100644
index 43aac12..0000000
--- a/core/modules/system/tests/themes/test_theme_twig/test_theme_twig.theme
+++ /dev/null
@@ -1,13 +0,0 @@
-<?php
-
-/**
- * @file
- * Theme to help test the Twig engine.
- */
-
-/**
- * Implements THEME_preprocess_twig_theme_test_php_variables().
- */
-function test_theme_twig_preprocess_twig_theme_test_php_variables(&$variables) {
-  $variables['php_values'] = _test_theme_twig_php_values();
-}
diff --git a/core/modules/system/theme.api.php b/core/modules/system/theme.api.php
index 399aca8..85f21d1 100644
--- a/core/modules/system/theme.api.php
+++ b/core/modules/system/theme.api.php
@@ -39,12 +39,8 @@
  * default implementations of theme hooks are named theme_HOOK. Default
  * template implementations are stored in the module directory.
  *
- * Drupal's default template renderer is a simple PHP parsing engine that
- * includes the template and stores the output. Drupal's theme engines
- * can provide alternate template engines, such as XTemplate, Smarty and
- * PHPTal. The most common template engine is PHPTemplate (included with
- * Drupal and implemented in phptemplate.engine, which uses Drupal's default
- * template renderer.
+ * Drupal's default template renderer is Twig. Drupal's theme engines can
+ * provide alternate template engines, such as XTemplate, Smarty and PHPTal.
  *
  * In order to create theme-specific implementations of these hooks, themes can
  * implement their own version of theme hooks, either as functions or templates.
@@ -53,10 +49,9 @@
  * its own version of hook_theme() to tell Drupal what it is implementing;
  * themes utilizing an engine will have their well-named theming functions
  * automatically registered for them. While this can vary based upon the theme
- * engine, the standard set by phptemplate is that theme functions should be
- * named THEMENAME_HOOK. For example, for Drupal's default theme (Bartik) to
- * implement the 'table' hook, the phptemplate.engine would find
- * bartik_table().
+ * engine, the standard is that theme functions should be named THEMENAME_HOOK.
+ * For example, for Drupal's default theme (Bartik) to implement the 'table'
+ * hook, the theme function should be called bartik_table().
  *
  * The theme system is described and defined in theme.inc.
  *
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/display/DisplayPluginBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/display/DisplayPluginBase.php
index 5d3b60e..346c4b7 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/display/DisplayPluginBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/display/DisplayPluginBase.php
@@ -1777,7 +1777,7 @@ public function buildOptionsForm(&$form, &$form_state) {
 
         // If there's a theme engine involved, we also need to know its extension
         // so we can give the proper filename.
-        $this->theme_extension = '.tpl.php';
+        $this->theme_extension = '.html.twig';
         if (isset($theme_engine)) {
           $extension_function = $theme_engine . '_extension';
           if (function_exists($extension_function)) {
diff --git a/core/themes/engines/twig/twig.engine b/core/themes/engines/twig/twig.engine
index 1ba60a5..b541cdc 100644
--- a/core/themes/engines/twig/twig.engine
+++ b/core/themes/engines/twig/twig.engine
@@ -10,7 +10,9 @@
  * Implements hook_theme().
  */
 function twig_theme($existing, $type, $theme, $path) {
-  return drupal_find_theme_templates($existing, '.html.twig', $path);
+  $templates = drupal_find_theme_functions($existing, array($theme));
+  $templates += drupal_find_theme_templates($existing, '.html.twig', $path);
+  return $templates;
 }
 
 /**