diff --git a/includes/theme.inc b/includes/theme.inc
index ebe5817..bd93af2 100644
--- a/includes/theme.inc
+++ b/includes/theme.inc
@@ -1029,6 +1029,7 @@ function theme($hook, $variables = array()) {
     }
     $hook = $candidate;
   }
+  $theme_hook_original = $hook;
 
   // If there's no implementation, check for more generic fallbacks. If there's
   // still no implementation, log an error and return an empty string.
@@ -1090,6 +1091,8 @@ function theme($hook, $variables = array()) {
     $variables += array($info['render element'] => array());
   }
 
+  $variables['theme_hook_original'] = $theme_hook_original;
+
   // Invoke the variable processors, if any. The processors may specify
   // alternate suggestions for which hook's template/function to use. If the
   // hook is a suggestion of a base hook, invoke the variable processors of
@@ -1198,7 +1201,12 @@ function theme($hook, $variables = array()) {
     if (isset($info['path'])) {
       $template_file = $info['path'] . '/' . $template_file;
     }
-    $output = $render_function($template_file, $variables);
+    if (variable_get('theme_debug_output', FALSE)) {
+      $output = _theme_render_template_debug($render_function, $template_file, $variables, $extension);
+    }
+    else {
+      $output = $render_function($template_file, $variables);
+    }
   }
 
   // restore path_to_theme()
@@ -1521,6 +1529,72 @@ function theme_render_template($template_file, $variables) {
 }
 
 /**
+ * Renders a template for any engine.
+ *
+ * Includes the possibility to get debug output by setting the
+ * theme_debug_output variable to TRUE.
+ *
+ * @param string $template_function
+ *   The function to call for rendering the template.
+ * @param string $template_file
+ *   The filename of the template to render.
+ * @param array $variables
+ *   A keyed array of variables that will appear in the output.
+ * @param string $extension
+ *   The extension used by the theme engine for template files.
+ *
+ * @return
+ *   The output generated by the template including debug information.
+ */
+function _theme_render_template_debug($template_function, $template_file, $variables, $extension) {
+  $output = array(
+    'debug_prefix' => '',
+    'debug_info' => '',
+    'rendered_markup' => call_user_func($template_function, $template_file, $variables),
+    'debug_suffix' => '',
+  );
+  $output['debug_prefix'] .= "\n\n<!-- THEME DEBUG -->";
+  $output['debug_prefix'] .= "\n<!-- CALL: theme('{$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'])) {
+    $variables['theme_hook_suggestions'] = array_reverse($variables['theme_hook_suggestions']);
+  }
+  // Add debug output for directly called suggestions like
+  // '#theme' => 'comment__node__article'.
+  if (strpos($variables['theme_hook_original'], '__') !== FALSE) {
+    $derived_suggestions[] = $hook = $variables['theme_hook_original'];
+    while ($pos = strrpos($hook, '__')) {
+      $hook = substr($hook, 0, $pos);
+      $derived_suggestions[] = $hook;
+    }
+    // Get the value of the base hook (last derived suggestion) and append it
+    // to the end of all theme suggestions.
+    $base_hook = array_pop($derived_suggestions);
+    $variables['theme_hook_suggestions'] = array_merge($derived_suggestions, $variables['theme_hook_suggestions']);
+    $variables['theme_hook_suggestions'][] = $base_hook;
+  }
+  if (!empty($variables['theme_hook_suggestions'])) {
+    $current_template = basename($template_file);
+    $suggestions = $variables['theme_hook_suggestions'];
+    // Only add the original theme hook if it wasn't a directly called
+    // suggestion.
+    if (strpos($variables['theme_hook_original'], '__') === FALSE) {
+      $suggestions[] = $variables['theme_hook_original'];
+    }
+    foreach ($suggestions as &$suggestion) {
+      $template = strtr($suggestion, '_', '-') . $extension;
+      $prefix = ($template == $current_template) ? 'x' : '*';
+      $suggestion = $prefix . ' ' . $template;
+    }
+    $output['debug_info'] .= "\n<!-- FILE NAME SUGGESTIONS:\n   " . implode("\n   ", $suggestions) . "\n-->";
+  }
+  $output['debug_info'] .= "\n<!-- BEGIN OUTPUT from '{$template_file}' -->\n";
+  $output['debug_suffix'] .= "\n<!-- END OUTPUT from '{$template_file}' -->\n\n";
+  return implode('', $output);
+}
+
+/**
  * Enables a given list of themes.
  *
  * @param $theme_list
diff --git a/modules/simpletest/tests/theme.test b/modules/simpletest/tests/theme.test
index 519a7a9..29c322e 100644
--- a/modules/simpletest/tests/theme.test
+++ b/modules/simpletest/tests/theme.test
@@ -500,3 +500,68 @@ class ThemeRegistryTestCase extends DrupalWebTestCase {
     $this->assertTrue($registry['theme_test_template_test_2'], 'Offset was returned correctly from the theme registry');
   }
 }
+
+/**
+ * Tests for theme debug markup.
+ */
+class ThemeDebugMarkupTestCase extends DrupalWebTestCase {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Theme debug markup',
+      'description' => 'Tests theme debug markup output.',
+      'group' => 'Theme',
+    );
+  }
+
+  function setUp() {
+    parent::setUp('theme_test', 'node');
+    theme_enable(array('test_theme'));
+  }
+
+  /**
+   * Tests debug markup added to template output.
+   */
+  function testDebugOutput() {
+    variable_set('theme_default', 'test_theme');
+    // Enable the debug output.
+    variable_set('theme_debug_output', TRUE);
+
+    $registry = theme_get_registry();
+    $extension = '.tpl.php';
+    // Populate array of templates.
+    $templates = drupal_find_theme_templates($registry, $extension, drupal_get_path('theme', 'test_theme'));
+    $templates += drupal_find_theme_templates($registry, $extension, drupal_get_path('module', 'node'));
+
+    // Create a node and test different features of the debug markup.
+    $node = $this->drupalCreateNode();
+    $this->drupalGet('node/' . $node->nid);
+    $this->assertRaw('<!-- THEME DEBUG -->', 'Twig debug markup found in theme output when debug is enabled.');
+    $this->assertRaw("CALL: theme('node')", 'Theme call information found.');
+    $this->assertRaw('x node--1' . $extension . PHP_EOL . '   * node--page' . $extension . PHP_EOL . '   * node' . $extension, '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;
+    $this->assertRaw("BEGIN OUTPUT from '$template_filename'", 'Full path to current template file found.');
+
+    // Create another node and make sure the template suggestions shown in the
+    // debug markup are correct.
+    $node2 = $this->drupalCreateNode();
+    $this->drupalGet('node/' . $node2->nid);
+    $this->assertRaw('* node--2' . $extension . PHP_EOL . '   * node--page' . $extension . PHP_EOL . '   x node' . $extension, '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);
+    $output = drupal_render($build);
+    $this->assertTrue(strpos($output, "CALL: theme('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 theme debug.
+    variable_set('theme_debug_output', FALSE);
+
+    $this->drupalGet('node/' . $node->nid);
+    $this->assertNoRaw('<!-- THEME DEBUG -->', 'Twig debug markup not found in theme output when debug is disabled.');
+  }
+
+}