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 638d8e5..cb154b7 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -41,7 +41,7 @@ function setUp() {
    * Render arrays that use a render element and templates (and hence call
    * template_preprocess()) must ensure the attributes at different occasions
    * are all merged correctly:
-   *   - $variables['attributes'] as passed in to _theme()
+   *   - $variables['attributes'] as passed in to drupal_render()
    *   - the render element's #attributes
    *   - any attributes set in the template's preprocessing function
    */
@@ -58,21 +58,23 @@ function testAttributeMerging() {
   }
 
   /**
-   * Test that _theme() returns expected data types.
+   * Test that drupal_render() returns expected data types.
    */
   function testThemeDataTypes() {
-    // theme_test_false is an implemented theme hook so _theme() should return a
+    // theme_test_false is an implemented theme hook so drupal_render() should return a
     // string, even though the theme function itself can return anything.
     $foos = array('null' => NULL, 'false' => FALSE, 'integer' => 1, 'string' => 'foo');
     foreach ($foos as $type => $example) {
-      $output = _theme('theme_test_foo', array('foo' => $example));
+      $theme_test_foo = array('#theme' => 'theme_test_foo', '#foo' => $example);
+      $output = drupal_render($theme_test_foo);
       $this->assertTrue(is_string($output), format_string('_theme() returns a string for data type !type.', array('!type' => $type)));
     }
 
-    // suggestionnotimplemented is not an implemented theme hook so _theme()
+    // suggestionnotimplemented is not an implemented theme hook so drupal_render()
     // should return FALSE instead of a string.
-    $output = _theme(array('suggestionnotimplemented'));
-    $this->assertIdentical($output, FALSE, '_theme() returns FALSE when a hook suggestion is not implemented.');
+    $suggestionnotimplemented = array('#theme' => 'suggestionnotimplemented');
+    $output = drupal_render($suggestionnotimplemented);
+    $this->assertIdentical($output, FALSE, 'drupal_render() returns FALSE when a hook suggestion is not implemented.');
   }
 
   /**
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 9c2105a..f3bd56d 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
@@ -34,7 +34,7 @@ public static function getInfo() {
    */
   function testTwigDebugMarkup() {
     $extension = twig_extension();
-    theme_enable(array('test_theme'));
+    \Drupal::service('theme_handler')->enable(array('test_theme'));
     \Drupal::config('system.theme')->set('default', 'test_theme')->save();
     // Enable debug, rebuild the service container, and clear all caches.
     $this->settingsSet('twig_debug', TRUE);
@@ -48,9 +48,10 @@ function testTwigDebugMarkup() {
 
     // Create a node and test different features of the debug markup.
     $node = $this->drupalCreateNode();
-    $output = _theme('node', node_view($node));
+    $node_output = entity_view($node, 'full');
+    $output = drupal_render($node_output);
     $this->assertTrue(strpos($output, '<!-- THEME DEBUG -->') !== FALSE, 'Twig debug markup found in theme output when debug is enabled.');
-    $this->assertTrue(strpos($output, "CALL: _theme('node')") !== FALSE, 'Theme call information found.');
+    $this->assertTrue(strpos($output, "CALL: drupal_render('node')") !== FALSE, 'Theme call information found.');
     $this->assertTrue(strpos($output, 'x node--1' . $extension . PHP_EOL . '   * node--page' . $extension . PHP_EOL . '   * node' . $extension) !== FALSE, 'Suggested template files found in order and node ID specific template shown as current template.');
     $template_filename = $templates['node__1']['path'] . '/' . $templates['node__1']['template'] . $extension;
     $this->assertTrue(strpos($output, "BEGIN OUTPUT from '$template_filename'") !== FALSE, 'Full path to current template file found.');
@@ -58,16 +59,17 @@ function testTwigDebugMarkup() {
     // Create another node and make sure the template suggestions shown in the
     // debug markup are correct.
     $node2 = $this->drupalCreateNode();
-    $output = _theme('node', node_view($node2));
+    $node2_output = entity_view($node2, 'full');
+    $output = drupal_render($node2_output);
     $this->assertTrue(strpos($output, '* node--2' . $extension . PHP_EOL . '   * node--page' . $extension . PHP_EOL . '   x node' . $extension) !== FALSE, 'Suggested template files found in order and base template shown as current template.');
 
     // Create another node and make sure the template suggestions shown in the
     // debug markup are correct.
     $node3 = $this->drupalCreateNode();
     $build = array('#theme' => 'node__foo__bar');
-    $build += node_view($node3);
+    $build += entity_view($node3, 'full');;
     $output = drupal_render($build);
-    $this->assertTrue(strpos($output, "CALL: _theme('node__foo__bar')") !== FALSE, 'Theme call information found.');
+    $this->assertTrue(strpos($output, "CALL: drupal_render('node__foo__bar')") !== FALSE, 'Theme call information found.');
     $this->assertTrue(strpos($output, '* node--foo--bar' . $extension . PHP_EOL . '   * node--foo' . $extension . PHP_EOL . '   * node--3' . $extension . PHP_EOL . '   * node--page' . $extension . PHP_EOL . '   x node' . $extension) !== FALSE, 'Suggested template files found in order and base template shown as current template.');
 
     // Disable debug, rebuild the service container, and clear all caches.
@@ -75,7 +77,8 @@ function testTwigDebugMarkup() {
     $this->rebuildContainer();
     $this->resetAll();
 
-    $output = _theme('node', node_view($node));
+    $node_output = entity_view($node, 'full');
+    $output = drupal_render($node_output);
     $this->assertFalse(strpos($output, '<!-- THEME DEBUG -->') !== FALSE, 'Twig debug markup not found in theme output when debug is disabled.');
   }
 
diff --git a/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php
index 31b25a0..df04a06 100644
--- a/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php
+++ b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/ThemeTestController.php
@@ -55,7 +55,8 @@ public function testInfoStylesheets() {
    *   A render array containing a theme override.
    */
   public function testTemplate() {
-    return _theme('theme_test_template_test');
+    $theme_test_template_test = array('#theme' => 'theme_test_template_test');
+    return drupal_render($theme_test_template_test);
   }
 
   /**
@@ -65,7 +66,8 @@ public function testTemplate() {
    *   An HTML string containing the themed output.
    */
   public function testSuggestion() {
-    return _theme(array('theme_test__suggestion', 'theme_test'), array());
+    $theme_test__suggestion = array('#theme' => array('theme_test__suggestion', 'theme_test'));
+    return drupal_render($theme_test__suggestion);
   }
 
 /**
diff --git a/core/modules/system/tests/modules/twig_theme_test/lib/Drupal/twig_theme_test/TwigThemeTestController.php b/core/modules/system/tests/modules/twig_theme_test/lib/Drupal/twig_theme_test/TwigThemeTestController.php
index ba2a718..b4817f5 100644
--- a/core/modules/system/tests/modules/twig_theme_test/lib/Drupal/twig_theme_test/TwigThemeTestController.php
+++ b/core/modules/system/tests/modules/twig_theme_test/lib/Drupal/twig_theme_test/TwigThemeTestController.php
@@ -16,7 +16,8 @@ class TwigThemeTestController {
    * Menu callback for testing PHP variables in a Twig template.
    */
   public function phpVariablesRender() {
-    return _theme('twig_theme_test_php_variables');
+    $twig_theme_test_php_variables = array('#theme' => 'twig_theme_test_php_variables');
+    return drupal_render($twig_theme_test_php_variables);
   }
 
   /**
diff --git a/core/themes/engines/twig/twig.engine b/core/themes/engines/twig/twig.engine
index 74de2b8..f398f7b 100644
--- a/core/themes/engines/twig/twig.engine
+++ b/core/themes/engines/twig/twig.engine
@@ -54,7 +54,7 @@ function twig_render_template($template_file, $variables) {
   );
   if (Settings::get('twig_debug', FALSE)) {
     $output['debug_prefix'] .= "\n\n<!-- THEME DEBUG -->";
-    $output['debug_prefix'] .= "\n<!-- CALL: _theme('{$variables['theme_hook_original']}') -->";
+    $output['debug_prefix'] .= "\n<!-- CALL: drupal_render('{$variables['theme_hook_original']}') -->";
     // If there are theme suggestions, reverse the array so more specific
     // suggestions are shown first.
     if (!empty($variables['theme_hook_suggestions'])) {