diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 13ce524..0384456 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -830,6 +830,7 @@ function drupal_find_base_themes($themes, $key, $used_keys = array()) {
  * engine, it overrides the default implementation of the 'page' theme hook by
  * containing a 'page.html.twig' file within its folder structure).
  *
+ * @todo Add/edit/remove these subsections with hook_theme_prepare().
  * @subsection sub_preprocess_templates Preprocessing for Template Files
  * If the implementation is a template file, several functions are called before
  * the template file is invoked to modify the $variables array. These make up
@@ -907,6 +908,8 @@ function drupal_find_base_themes($themes, $key, $used_keys = array()) {
  * @see themeable
  * @see hook_theme()
  * @see template_preprocess()
+ *
+ * @throws Exception
  */
 function theme($hook, $variables = array()) {
   static $default_attributes;
@@ -1058,8 +1061,8 @@ function theme($hook, $variables = array()) {

   // Provide immutable context for subsequent hooks.
   $context_original = array(
-    'theme_id' => $hook,
-    'original_theme_id' => $original_hook,
+    'theme_hook' => $hook,
+    'original_theme_hook' => $original_hook,
     'suggestions' => $suggestions,
   );

@@ -1074,7 +1077,7 @@ function theme($hook, $variables = array()) {
     $prepare_hooks[] = 'theme_prepare_' . $original_hook;
   }

-  // Invokes hook_theme_prepare(), hook_theme_prepare_THEME_ID() for enabled
+  // Invokes hook_theme_prepare(), hook_theme_prepare_THEME_HOOK() for enabled
   // modules.
   foreach ($prepare_hooks as $prepare_hook) {
     $context = $context_original;
@@ -1090,8 +1093,7 @@ function theme($hook, $variables = array()) {
     $variables = NestedArray::mergeDeep($variables, (array) Drupal::moduleHandler()->themeInvoke($prepare_hook, array($context)));
   }

-  // Invokes preprocess functions.
-  // @todo Deprecate @see https://drupal.org/node/2060773
+  // Invokes preprocess functions (deprecated).
   if (isset($info['preprocess functions'])) {
     foreach ($info['preprocess functions'] as $preprocessor_function) {
       if (function_exists($preprocessor_function)) {
diff --git a/core/modules/system/theme.api.php b/core/modules/system/theme.api.php
index 34758da..3ca5861 100644
--- a/core/modules/system/theme.api.php
+++ b/core/modules/system/theme.api.php
@@ -104,9 +104,15 @@ function hook_form_system_theme_settings_alter(&$form, &$form_state) {
  *   The variables array (modify in place).
  * @param $hook
  *   The name of the theme hook.
+ *
+ * @deprecated
+ * @see hook_theme_prepare()
  */
 function hook_preprocess(&$variables, $hook) {
- static $hooks;
+  // This hook has been deprecated, do not use it. Please use instead:
+  // hook_theme_prepare().
+
+  static $hooks;

   // Add contextual links to the variables, if the user has permission.

@@ -150,14 +156,138 @@ function hook_preprocess(&$variables, $hook) {
  *
  * @param $variables
  *   The variables array (modify in place).
+ *
+ * @deprecated
+ * @see hook_theme_prepare_SUGGESTION()
  */
 function hook_preprocess_HOOK(&$variables) {
+  // This hook has been deprecated, do not use it. Please use instead:
+  // hook_theme_prepare_SUGGESTION().
+
   // This example is from rdf_preprocess_image(). It adds an RDF attribute
   // to the image hook's variables.
   $variables['attributes']['typeof'] = array('foaf:Image');
 }

 /**
+ * Prepares theme variables for templates.
+ *
+ * This hook allows modules to prepare theme variables for theme templates.
+ * It is called for all theme hooks implemented as templates, but not for theme
+ * hooks implemented as functions. hook_preprocess_HOOK() can be used to
+ * preprocess variables for a specific theme hook, whether implemented as a
+ * template or function.
+ *
+ * For more detailed information, see theme().
+ *
+ * @param array $variables
+ *   The variables array (modify in place).
+ * @param array $context
+ *   The immutable context array.
+ */
+function hook_theme_prepare(&$variables, $context) {
+  static $hooks;
+
+  // Add contextual links to the variables, if the user has permission.
+  if (!user_access('access contextual links')) {
+    return;
+  }
+
+  if (!isset($hooks)) {
+    $hooks = theme_get_registry();
+  }
+
+  // Retrieve the original theme hook from context.
+  $hook = $context['original_theme_hook'];
+
+  // Determine the primary theme function argument.
+  if (isset($hooks[$hook]['variables'])) {
+    $keys = array_keys($hooks[$hook]['variables']);
+    $key = $keys[0];
+  }
+  else {
+    $key = $hooks[$hook]['render element'];
+  }
+
+  if (isset($key) && !empty($variables[$key])) {
+    $element = $variables[$key];
+  }
+
+  if (isset($element) && is_array($element) && !empty($element['#contextual_links'])) {
+    $variables['title_suffix']['contextual_links'] = contextual_links_view($element);
+    if (!empty($variables['title_suffix']['contextual_links'])) {
+      $variables['attributes']['class'][] = 'contextual-links-region';
+    }
+  }
+}
+
+/**
+ * Alters the prepared theme variables used in templates.
+ *
+ * This hook allows any module or theme to add new variables or reorder/remove
+ * existing variables provided by earlier invocations of hook_theme_prepare() or
+ * hook_theme_prepare_THEME_HOOK().
+ *
+ * For more detailed information, see theme().
+ *
+ * @param array $variables
+ *   The variables array (modify in place).
+ * @param array $context
+ *   The immutable context array.
+ */
+function hook_theme_prepare_alter(&$variables, $context) {
+  // Replace the name of the user whom authored the node.
+  if ($context['theme_hook'] === 'node') {
+    $variables['name'] = t('Anonymous Unicorn');
+  }
+}
+
+/**
+ * Prepares variables used in templates for a specific theme suggestion.
+ *
+ * This hook allows modules to prepare theme variables for a specific theme
+ * suggestion. It should only be used when it is needed to override or add to
+ * theme preparation for a theme hook it did not define.
+ *
+ * THEME_HOOK can be either the base theme hook or a more specific suggestion
+ * using the double-underscore ('__') notation.
+ *
+ * For more detailed information, see theme().
+ *
+ * @param $variables
+ *   The variables array (modify in place).
+ * @param array $context
+ *   The immutable context array.
+ */
+function hook_theme_prepare_THEME_HOOK(&$variables, $context) {
+  // This example is from rdf_theme_prepare_image(). It adds an RDF attribute
+  // to the image hook's variables.
+  $variables['attributes']['typeof'] = array('foaf:Image');
+}
+
+/**
+ * Alters prepared variables used in templates for a specific theme suggestion.
+ *
+ * This hook allows any module or theme to add new variables or reorder/remove
+ * existing variables provided by earlier invocations of hook_theme_prepare() or
+ * hook_theme_prepare_THEME_HOOK().
+ *
+ * THEME_HOOK can be either the base theme hook or a more specific suggestion
+ * using the double-underscore ('__') notation.
+ *
+ * For more detailed information, see theme().
+ *
+ * @param array $variables
+ *   The variables array (modify in place).
+ * @param array $context
+ *   The immutable context array.
+ */
+function hook_theme_prepare_THEME_HOOK_alter(&$variables, $context) {
+  // Replace the name of the user whom authored the node.
+  $variables['name'] = t('Anonymous Unicorn');
+}
+
+/**
  * Provides alternate named suggestions for a specific theme hook.
  *
  * This hook allows the module implementing hook_theme() for a theme hook to
@@ -173,8 +303,8 @@ function hook_preprocess_HOOK(&$variables) {
  * @todo Add @code sample.
  *
  * @param array $variables
- *   An array of variables passed to the theme hook. Note that this hook is
- *   invoked before any preprocessing.
+ *   An array of variables passed to the theme hook. NOTE: this hook is invoked
+ *   prior to hook_theme_prepare(). Some variables may not always be available.
  *
  * @return array
  *   An array of theme suggestions.