diff --git a/includes/theme.inc b/includes/theme.inc
--- 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'] = $theme_hook_original;
+
   // Invoke the variable processors, if any. The processors may specify
   // alternate suggestions for which hook's template/function to use. If the
   // hook is a suggestion of a base hook, invoke the variable processors of
@@ -1198,7 +1201,7 @@ function theme($hook, $variables = array()) {
     if (isset($info['path'])) {
       $template_file = $info['path'] . '/' . $template_file;
     }
-    $output = $render_function($template_file, $variables);
+    $output = _theme_render_template_debug($render_function, $template_file, $variables);
   }
 
   // restore path_to_theme()
@@ -1521,6 +1524,73 @@ 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 $template_function
+ *   The function to call for rendering the template.
+ * @param $template_file
+ *   The filename of the template to render.
+ * @param $variables
+ *   A keyed array of variables that will appear in the output.
+ *
+ * @return
+ *   The output generated by the template.
+ */
+function _theme_render_template_debug($template_function, $template_file, $variables) {
+  $output = array(
+    'debug_prefix' => '',
+    'debug_info' => '',
+    'rendered_markup' => call_user_func($template_function, $template_file, $variables),
+    'debug_suffix' => '',
+  );
+  if (variable_get('theme_debug_output', FALSE)) {
+    $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'])) {
+      $extension = '.tpl.php';
+      $current_template = basename($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