diff --git a/core/modules/system/lib/Drupal/system/Controller/SystemInfoController.php b/core/modules/system/lib/Drupal/system/Controller/SystemInfoController.php
index af48c1f..e67453d 100644
--- a/core/modules/system/lib/Drupal/system/Controller/SystemInfoController.php
+++ b/core/modules/system/lib/Drupal/system/Controller/SystemInfoController.php
@@ -53,7 +53,11 @@ public function __construct(SystemManager $systemManager) {
   public function status() {
     $requirements = $this->systemManager->listRequirements();
     $this->systemManager->fixAnonymousUid();
-    return theme('status_report', array('requirements' => $requirements));
+    $status_report = array(
+      '#theme' => 'status_report',
+      '#requirements' => $requirements,
+    );
+    return drupal_render($status_report);
   }
 
   /**
diff --git a/core/modules/system/lib/Drupal/system/Plugin/Block/SystemPoweredByBlock.php b/core/modules/system/lib/Drupal/system/Plugin/Block/SystemPoweredByBlock.php
index 7edd34b..b5fe6da 100644
--- a/core/modules/system/lib/Drupal/system/Plugin/Block/SystemPoweredByBlock.php
+++ b/core/modules/system/lib/Drupal/system/Plugin/Block/SystemPoweredByBlock.php
@@ -26,8 +26,9 @@ class SystemPoweredByBlock extends BlockBase {
    * {@inheritdoc}
    */
   public function build() {
+    $system_powered_by = array('#theme' => 'system_powered_by');
     return array(
-      '#children' => theme('system_powered_by'),
+      '#children' => drupal_render($system_powered_by),
     );
   }
 
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 072bfb0..8c03477 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -41,19 +41,17 @@ function setUp() {
    * Render arrays that use a render element and templates (and hence call
    * template_preprocess()) must ensure the attributes at different occassions
    * 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
    */
   function testAttributeMerging() {
-    $output = theme('theme_test_render_element', array(
-      'elements' => array(
-        '#attributes' => array('data-foo' => 'bar'),
-      ),
-      'attributes' => array(
-        'id' => 'bazinga',
-      ),
-    ));
+    $theme_test_render_element = array(
+      '#theme' => 'theme_test_render_element',
+      '#elements' => array('#attributes' => array('data-foo' => 'bar')),
+      '#attributes' => array('id' => 'bazinga'),
+    );
+    $output = drupal_render($theme_test_render_element);
     $this->assertIdentical($output, '<div id="bazinga" data-foo="bar" data-variables-are-preprocessed></div>' . "\n");
   }
 
@@ -196,44 +194,47 @@ function testThemeGetSetting() {
    * Ensures the theme registry is rebuilt when modules are disabled/enabled.
    */
   function testRegistryRebuild() {
-    $this->assertIdentical(theme('theme_test_foo', array('foo' => 'a')), 'a', 'The theme registry contains theme_test_foo.');
+    $theme_test_foo = array( '#theme' => 'theme_test_foo', '#foo' => 'a' );
+    $this->assertIdentical(drupal_render($theme_test_foo), 'a', 'The theme registry contains theme_test_foo.');
 
     module_disable(array('theme_test'), FALSE);
     // After enabling/disabling a module during a test, we need to rebuild the
-    // container and ensure the extension handler is loaded, otherwise theme()
-    // throws an exception.
+    // container and ensure the extension handler is loaded, otherwise
+    // drupal_render() throws an exception.
     $this->rebuildContainer();
     $this->container->get('module_handler')->loadAll();
-    $this->assertIdentical(theme('theme_test_foo', array('foo' => 'b')), FALSE, 'The theme registry does not contain theme_test_foo, because the module is disabled.');
+    $theme_test_foo = array('#theme' => 'theme_test_foo', '#foo' => 'b');
+    $this->assertIdentical(drupal_render($theme_test_foo), FALSE, 'The theme registry does not contain theme_test_foo, because the module is disabled.');
 
     module_enable(array('theme_test'), FALSE);
     // After enabling/disabling a module during a test, we need to rebuild the
-    // container and ensure the extension handler is loaded, otherwise theme()
-    // throws an exception.
+    // container and ensure the extension handler is loaded, otherwise
+    // drupal_render() throws an exception.
     $this->rebuildContainer();
     $this->container->get('module_handler')->loadAll();
-    $this->assertIdentical(theme('theme_test_foo', array('foo' => 'c')), 'c', 'The theme registry contains theme_test_foo again after re-enabling the module.');
+    $theme_test_foo = array('#theme' => 'theme_test_foo', '#foo' => 'c');
+    $this->assertIdentical(drupal_render($theme_test_foo), 'c', 'The theme registry contains theme_test_foo again after re-enabling the module.');
   }
 
   /**
    * Tests child element rendering for 'render element' theme hooks.
    */
   function testDrupalRenderChildren() {
-    $element = array(
+    $theme_test_render_element_children = array(
       '#theme' => 'theme_test_render_element_children',
       'child' => array(
         '#markup' => 'Foo',
       ),
     );
-    $this->assertIdentical(theme('theme_test_render_element_children', $element), 'Foo', 'drupal_render() avoids #theme recursion loop when rendering a render element.');
+    $this->assertIdentical(drupal_render($theme_test_render_element_children), 'Foo', 'drupal_render() avoids #theme recursion loop when rendering a render element.');
 
-    $element = array(
+    $theme_test_render_element_children = array(
       '#theme_wrappers' => array('theme_test_render_element_children'),
       'child' => array(
         '#markup' => 'Foo',
       ),
     );
-    $this->assertIdentical(theme('theme_test_render_element_children', $element), 'Foo', 'drupal_render() avoids #theme_wrappers recursion loop when rendering a render element.');
+    $this->assertIdentical(drupal_render($theme_test_render_element_children), 'Foo', 'drupal_render() avoids #theme_wrappers recursion loop when rendering a render element.');
   }
 
   /**
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 38e183b..c049dd0 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/TwigDebugMarkupTest.php
@@ -52,7 +52,7 @@ function testTwigDebugMarkup() {
 
     // Create a node and test different features of the debug markup.
     $node = $this->drupalCreateNode();
-    $output = theme('node', node_view($node));
+    $output = drupal_render(node_view($node));
     $this->assertTrue(strpos($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, 'x node--1' . $extension) !== FALSE, 'Node ID specific template shown as current template.');
@@ -63,7 +63,7 @@ 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));
+    $output = drupal_render(node_view($node2));
     $this->assertTrue(strpos($output, '* node--2' . $extension) !== FALSE, 'Node ID specific template suggestion found.');
     $this->assertTrue(strpos($output, 'x node' . $extension) !== FALSE, 'Base template file shown as current template.');
 
@@ -72,7 +72,7 @@ function testTwigDebugMarkup() {
     $this->rebuildContainer();
     $this->resetAll();
 
-    $output = theme('node', node_view($node));
+    $output = drupal_render(node_view($node));
     $this->assertFalse(strpos($output, '<!-- THEME DEBUG -->') !== FALSE, 'Twig debug markup not found in theme output when debug is disabled.');
   }
 
diff --git a/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
index 957db86..e72a57f 100644
--- a/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -48,8 +48,12 @@ function system_admin_config_page() {
           unset($item['localized_options']['attributes']['title']);
         }
         $block = $item;
+        $admin_block_content = array(
+          '#theme' => 'admin_block_content',
+          '#content' => system_admin_menu_block($item),
+        );
         $block['content'] = '';
-        $block['content'] .= theme('admin_block_content', array('content' => system_admin_menu_block($item)));
+        $block['content'] .= drupal_render($admin_block_content);
         if (!empty($block['content'])) {
           $block['show'] = TRUE;
         }
@@ -62,7 +66,11 @@ function system_admin_config_page() {
   }
   if ($blocks) {
     ksort($blocks);
-    return theme('admin_page', array('blocks' => $blocks));
+    $admin_page = array(
+      '#theme' => 'admin_page',
+      '#blocks' => $blocks,
+    );
+    return drupal_render($admin_page);
   }
   else {
     return t('You do not have any administrative items.');
@@ -82,8 +90,9 @@ function system_admin_config_page() {
  */
 function system_admin_menu_block_page() {
   $item = menu_get_item();
-  if ($content = system_admin_menu_block($item)) {
-    $output = theme('admin_block_content', array('content' => $content));
+  $admin_block_content = array('#theme' => 'admin_block_content');
+  if ($admin_block_content['#content'] = system_admin_menu_block($item)) {
+    $output = drupal_render($admin_block_content);
   }
   else {
     $output = t('You do not have any administrative items.');
@@ -218,7 +227,12 @@ function system_themes_page() {
   drupal_alter('system_themes_page', $theme_groups);
 
   $admin_form = drupal_get_form('system_themes_admin_form', $admin_theme_options);
-  return theme('system_themes_page', array('theme_groups' => $theme_groups, 'theme_group_titles' => $theme_group_titles)) . drupal_render($admin_form);
+  $system_themes_page = array(
+    '#theme' => 'system_themes_page',
+    '#theme_groups' => $theme_groups,
+    '#theme_group_titles' => $theme_group_titles,
+  );
+  return drupal_render($system_themes_page) . drupal_render($admin_form);
 }
 
 /**
@@ -614,9 +628,15 @@ function _system_modules_build_row($info, $extra) {
   }
   else {
     $form['enable'] = array(
-      '#markup' =>  theme('image', array('uri' => 'core/misc/watchdog-error.png', 'alt' => $status_short, 'title' => $status_short)),
+      '#theme' => 'image',
+      '#uri' => 'core/misc/watchdog-error.png',
+      '#alt' => $status_short,
+      '#title' => $status_short,
+    );
+    $form['description'] = array(
+      '#theme' => 'system_modules_incompatible',
+      '#message' => $status_long,
     );
-    $form['description']['#markup'] .= theme('system_modules_incompatible', array('message' => $status_long));
   }
 
   // Build operation links.
@@ -983,7 +1003,11 @@ function theme_admin_page($variables) {
   $container = array();
 
   foreach ($blocks as $block) {
-    if ($block_output = theme('admin_block', array('block' => $block))) {
+    $admin_block = array(
+      '#theme' => 'admin_block',
+      '#block' => $block,
+    );
+    if ($block_output = drupal_render($admin_block)) {
       if (empty($block['position'])) {
         // perform automatic striping.
         $block['position'] = ++$stripe % 2 ? 'left' : 'right';
@@ -995,8 +1019,9 @@ function theme_admin_page($variables) {
     }
   }
 
+  $system_compact_link = array('#theme' => 'system_compact_link');
   $output = '<div class="admin clearfix">';
-  $output .= theme('system_compact_link');
+  $output .= drupal_render($system_compact_link);
 
   foreach ($container as $id => $data) {
     $output .= '<div class="' . $id . ' clearfix">';
@@ -1027,16 +1052,24 @@ function theme_system_admin_index($variables) {
   // Iterate over all modules.
   foreach ($menu_items as $module => $block) {
     list($description, $items) = $block;
+    $admin_block_content = array(
+      '#theme' => 'admin_block_content',
+      '#content' => $items,
+    );
+    $admin_block = array(
+      '#theme' => 'admin_block',
+      '#block' => $block,
+    );
 
     // Output links.
     if (count($items)) {
       $block = array();
       $block['title'] = $module;
-      $block['content'] = theme('admin_block_content', array('content' => $items));
+      $block['content'] = drupal_render($admin_block_content);
       $block['description'] = t($description);
       $block['show'] = TRUE;
 
-      if ($block_output = theme('admin_block', array('block' => $block))) {
+      if ($block_output = drupal_render($admin_block)) {
         if (!isset($block['position'])) {
           // Perform automatic striping.
           $block['position'] = $position;
@@ -1047,8 +1080,9 @@ function theme_system_admin_index($variables) {
     }
   }
 
+  $system_compact_link = array('#theme' => 'system_compact_link');
   $output = '<div class="admin clearfix">';
-  $output .= theme('system_compact_link');
+  $output .= drupal_render($system_compact_link);
   foreach ($container as $id => $data) {
     $output .= '<div class="' . $id . ' clearfix">';
     $output .= $data;
@@ -1281,7 +1315,19 @@ function theme_system_themes_page($variables) {
     foreach ($theme_groups[$state] as $theme) {
 
       // Theme the screenshot.
-      $screenshot = $theme->screenshot ? theme('image', $theme->screenshot) : '<div class="no-screenshot"><div class="no-screenshot__text">' . t('no screenshot') . '</div></div>';
+      if ($theme->screenshot) {
+        $image = array(
+          '#theme' => 'image',
+          '#uri' => $theme->screenshot['uri'],
+          '#alt' => $theme->screenshot['alt'],
+          '#title' => $theme->screenshot['title'],
+          '#attributes' => $theme->screenshot['attributes'],
+        );
+        $screenshot = drupal_render($image);
+      }
+      else {
+        $screenshot = '<div class="no-screenshot"><div class="no-screenshot__text">' . t('no screenshot') . '</div></div>';
+      }
 
       // Localize the theme description.
       $description = t($theme->info['description']);
@@ -1309,7 +1355,14 @@ function theme_system_themes_page($variables) {
         $output .= '<div class="incompatible">' . t('This theme requires the theme engine @theme_engine to operate correctly.', array('@theme_engine' => $theme->info['engine'])) . '</div>';
       }
       else {
-        $output .= theme('links', array('links' => $theme->operations, 'attributes' => array('class' => array('operations', 'clearfix'))));
+        $links = array(
+          '#theme' => 'links',
+          '#links' => $theme->operations,
+          '#attributes' => array(
+            'class' => array('operations', 'clearfix'),
+          ),
+        );
+        $output .= drupal_render($links);
       }
       $output .= '</div></div>';
     }
diff --git a/core/modules/system/system.api.php b/core/modules/system/system.api.php
index 1e1bae3..d4388b2 100644
--- a/core/modules/system/system.api.php
+++ b/core/modules/system/system.api.php
@@ -410,7 +410,8 @@ function hook_css_alter(&$css) {
  */
 function hook_ajax_render_alter($commands) {
   // Inject any new status messages into the content area.
-  $commands[] = ajax_command_prepend('#block-system-main .content', theme('status_messages'));
+  $status_messages = array('#theme' => 'status_messages');
+  $commands[] = ajax_command_prepend('#block-system-main .content', drupal_render($status_messages));
 }
 
 /**
diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index f10d86b..2fa9930 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -130,7 +130,11 @@ function system_requirements($phase) {
       '@system_requirements' => 'http://drupal.org/requirements',
     ));
 
-    $description .= theme('item_list', array('items' => $missing_extensions));
+    $item_list = array(
+      '#theme' => 'item_list',
+      '#items' => $missing_extensions,
+    );
+    $description .= drupal_render($item_list);
 
     $requirements['php_extensions']['value'] = t('Disabled');
     $requirements['php_extensions']['severity'] = REQUIREMENT_ERROR;
@@ -245,7 +249,11 @@ function system_requirements($phase) {
         $description = $conf_errors[0];
       }
       else {
-        $description = theme('item_list', array('items' => $conf_errors));
+        $item_list = array(
+          '#theme' => 'item_list',
+          '#items' => $conf_errors,
+        );
+        $description = drupal_render($item_list);
       }
       $requirements['settings.php'] = array(
         'value' => t('Not protected'),
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 0df34a0..6f6b2bc 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -3592,7 +3592,12 @@ function theme_exposed_filters($variables) {
     foreach (element_children($form['current']) as $key) {
       $items[] = $form['current'][$key];
     }
-    $output .= theme('item_list', array('items' => $items, 'attributes' => array('class' => array('clearfix', 'current-filters'))));
+    $item_list = array(
+      '#theme' => 'item_list',
+      '#items' => $items,
+      '#attributes' => array('class' => array('clearfix', 'current-filters')),
+    );
+    $output .= drupal_render($item_list);
   }
 
   $output .= drupal_render_children($form);
diff --git a/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/EventSubscriber/ThemeTestSubscriber.php b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/EventSubscriber/ThemeTestSubscriber.php
index bea8b13..3bfe755 100644
--- a/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/EventSubscriber/ThemeTestSubscriber.php
+++ b/core/modules/system/tests/modules/theme_test/lib/Drupal/theme_test/EventSubscriber/ThemeTestSubscriber.php
@@ -34,7 +34,12 @@ public function onRequest(GetResponseEvent $event) {
       // theme_test_request_listener_page_callback() to test that even when the
       // theme system is initialized this early, it is still capable of
       // returning output and theming the page as a whole.
-      $GLOBALS['theme_test_output'] = theme('more_link', array('url' => 'user', 'title' => 'Themed output generated in a KernelEvents::REQUEST listener'));
+      $more_link = array(
+        '#theme' => 'more_link',
+        '#url' => 'user',
+        '#title' => 'Themed output generated in a KernelEvents::REQUEST listener',
+      );
+      $GLOBALS['theme_test_output'] = drupal_render($more_link);
     }
     if (strpos($current_path, 'user/autocomplete') === 0) {
       // Register a fake registry loading callback. If it gets called by
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 ca8439d..17dedaf 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -93,7 +93,8 @@ function theme_test_request_listener_page_callback() {
  * Menu callback for testing template overridding based on filename.
  */
 function theme_test_template_test_page_callback() {
-  return theme('theme_test_template_test');
+  $theme_test_template_test = array('#theme' => 'theme_test_template_test');
+  return drupal_render($theme_test_template_test);
 }
 
 /**
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 ef5fb70..5299481 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
@@ -26,7 +26,8 @@ public static function create(ContainerInterface $container) {
    * 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/modules/system/theme.api.php b/core/modules/system/theme.api.php
index aedc6b0..169e1aa 100644
--- a/core/modules/system/theme.api.php
+++ b/core/modules/system/theme.api.php
@@ -186,7 +186,13 @@ function hook_process(&$variables, $hook) {
         'variable_name' => $variable_name,
         'variables' => $variables,
       );
-      $variables[$variable_name] = theme('rdf_template_variable_wrapper', array('content' => $variables[$variable_name], 'attributes' => $attributes, 'context' => $context));
+      $rdf_template_variable_wrapper = array(
+        '#theme' => 'rdf_template_variable_manager',
+        '#content' => $variables[$variable_name],
+        '#attributes' => $attributes,
+        '#context' => $context,
+      );
+      $variables[$variable_name] = drupal_render($rdf_template_variable_wrapper);
     }
   }
 }