diff --git a/core/lib/Drupal/Core/EventSubscriber/ConfigImportSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/ConfigImportSubscriber.php
index abd6d6d..132559c 100644
--- a/core/lib/Drupal/Core/EventSubscriber/ConfigImportSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/ConfigImportSubscriber.php
@@ -58,27 +58,61 @@ public function onConfigImporterValidate(ConfigImporterEvent $event) {
     }
     $config_importer = $event->getConfigImporter();
     if ($config_importer->getStorageComparer()->getSourceStorage()->exists('core.extension')) {
-      // Modules are already validated in the ConfigImporter.
-      // @see \Drupal\Core\Config\ConfigImporter::createExtensionChangelist()
-      $this->validateThemeInstalls($config_importer);
+      $this->validateModules($config_importer);
+      $this->validateThemes($config_importer);
       $this->validateDependencies($config_importer);
     }
   }
 
   /**
-   * Validates all themes being installed during a configuration import exist.
+   * Validates module installations and uninstallations.
    *
    * @param \Drupal\Core\Config\ConfigImporter $config_importer
    *   The configuration importer.
    */
-  protected function validateThemeInstalls(ConfigImporter $config_importer) {
+  protected function validateModules($config_importer) {
+    $core_extension = $config_importer->getStorageComparer()->getSourceStorage()->read('core.extension');
+    // Get a list of modules with dependency weights as values.
+    $module_data = system_rebuild_module_data();
+    $nonexistent_modules = array_diff(array_keys($core_extension['module']), array_keys($module_data));
+    foreach ($nonexistent_modules as $module) {
+      $config_importer->logError($this->t('Unable to install module %module since it does not exist.', array('%module' => $module)));
+    }
+
+    // Ensure that module dependencies are respected.
+    $uninstalls = $config_importer->getExtensionChangelist('module', 'uninstall');
+    foreach ($uninstalls as $module) {
+      foreach (array_keys($module_data[$module]->required_by) as $dependent_module) {
+        if ($module_data[$dependent_module]->status && !in_array($dependent_module, $uninstalls, TRUE)) {
+          $config_importer->logError($this->t('Unable to uninstall module %module since %dependent_module is installed.', array('%module' => $module, '%dependent_module' => $dependent_module)));
+        }
+      }
+    }
+  }
+
+  /**
+   * Validates theme installations and uninstallations.
+   *
+   * @param \Drupal\Core\Config\ConfigImporter $config_importer
+   *   The configuration importer.
+   */
+  protected function validateThemes(ConfigImporter $config_importer) {
     // Get all themes including those that are not installed.
-    $themes = $this->themeHandler->rebuildThemeData();
+    $theme_data = $this->themeHandler->rebuildThemeData();
     foreach ($config_importer->getExtensionChangelist('theme', 'install') as $theme) {
-      if (!isset($themes[$theme])) {
+      if (!isset($theme_data[$theme])) {
         $config_importer->logError($this->t('Unable to install theme %theme since it does not exist.', array('%theme' => $theme)));
       }
     }
+
+    $uninstalls = $config_importer->getExtensionChangelist('theme', 'uninstall');
+    foreach ($uninstalls as $theme) {
+      foreach (array_keys($theme_data[$theme]->required_by) as $dependent_theme) {
+        if ($theme_data[$dependent_theme]->status && !in_array($dependent_theme, $uninstalls, TRUE)) {
+          $config_importer->logError($this->t('Unable to uninstall theme %theme since %dependent_theme is installed.', array('%theme' => $theme, '%dependent_theme' => $dependent_theme)));
+        }
+      }
+    }
   }
 
   /**
diff --git a/core/modules/config/src/Form/ConfigSync.php b/core/modules/config/src/Form/ConfigSync.php
index b97218b..7624f0f 100644
--- a/core/modules/config/src/Form/ConfigSync.php
+++ b/core/modules/config/src/Form/ConfigSync.php
@@ -340,7 +340,7 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
       }
       catch (ConfigImporterException $e) {
         // There are validation errors.
-        drupal_set_message($this->t('The configuration synchronization failed validation.'));
+        drupal_set_message($this->t('The configuration synchronization failed validation.'), 'error');
         foreach ($config_importer->getErrors() as $message) {
           drupal_set_message($message, 'error');
         }
diff --git a/core/modules/config/src/Tests/ConfigImportUITest.php b/core/modules/config/src/Tests/ConfigImportUITest.php
index ba44925..ed0558b 100644
--- a/core/modules/config/src/Tests/ConfigImportUITest.php
+++ b/core/modules/config/src/Tests/ConfigImportUITest.php
@@ -453,4 +453,35 @@ public function testEntityBundleDelete() {
     $this->assertNoText(format_string('core.entity_form_display.node.@type.default', array('@type' => $node_type->id())));
   }
 
+  /**
+   * Tests config importer cannot uninstall extensions which are depended on.
+   *
+   * @see \Drupal\Core\EventSubscriber\ConfigImportSubscriber
+   */
+  public function testExtensionValidation() {
+    \Drupal::service('module_installer')->install(['node']);
+    \Drupal::service('theme_handler')->install(['bartik']);
+    $this->rebuildContainer();
+
+    $staging = $this->container->get('config.storage.staging');
+    $this->copyConfig($this->container->get('config.storage'), $staging);
+    $core = $staging->read('core.extension');
+    // Node depends on text.
+    unset($core['module']['text']);
+    // Bartik depends on classy.
+    unset($core['theme']['classy']);
+    // This module does not exist.
+    $core['module']['does_not_exist'] = 0;
+    // This theme does not exist.
+    $core['theme']['does_not_exist'] = 0;
+    $staging->write('core.extension', $core);
+
+    $this->drupalPostForm('admin/config/development/configuration', array(), t('Import all'));
+    $this->assertText('The configuration synchronization failed validation.');
+    $this->assertText('Unable to uninstall module text since node is installed.');
+    $this->assertText('Unable to uninstall theme classy since bartik is installed.');
+    $this->assertText('Unable to install module does_not_exist since it does not exist.');
+    $this->assertText('Unable to install theme does_not_exist since it does not exist.');
+  }
+
 }