diff --git a/core/lib/Drupal/Core/Config/Config.php b/core/lib/Drupal/Core/Config/Config.php
index e000200..fc60369 100644
--- a/core/lib/Drupal/Core/Config/Config.php
+++ b/core/lib/Drupal/Core/Config/Config.php
@@ -137,7 +137,7 @@ class Config extends DependencySerialization {
    * @param \Drupal\Core\Language\Language $language
    *   The language object used to override configuration data.
    */
-  public function __construct($name, StorageInterface $storage, EventDispatcherInterface $event_dispatcher, TypedConfigManager $typed_config, Language $language = NULL) {
+  public function __construct($name, StorageInterface $storage, EventDispatcherInterface $event_dispatcher, TypedConfigManagerInterface $typed_config, Language $language = NULL) {
     $this->name = $name;
     $this->storage = $storage;
     $this->eventDispatcher = $event_dispatcher;
@@ -201,13 +201,13 @@ public function setName($name) {
   public static function validateName($name) {
     // The name must be namespaced by owner.
     if (strpos($name, '.') === FALSE) {
-      throw new ConfigNameException(format_string('Missing namespace in Config object name @name.', array(
+      throw new ConfigNameException(String::format('Missing namespace in Config object name @name.', array(
         '@name' => $name,
       )));
     }
     // The name must be shorter than Config::MAX_NAME_LENGTH characters.
     if (strlen($name) > self::MAX_NAME_LENGTH) {
-      throw new ConfigNameException(format_string('Config object name @name exceeds maximum allowed length of @length characters.', array(
+      throw new ConfigNameException(String::format('Config object name @name exceeds maximum allowed length of @length characters.', array(
         '@name' => $name,
         '@length' => self::MAX_NAME_LENGTH,
       )));
@@ -216,7 +216,7 @@ public static function validateName($name) {
     // The name must not contain any of the following characters:
     // : ? * < > " ' / \
     if (preg_match('/[:?*<>"\'\/\\\\]/', $name)) {
-      throw new ConfigNameException(format_string('Invalid character in Config object name @name.', array(
+      throw new ConfigNameException(String::format('Invalid character in Config object name @name.', array(
         '@name' => $name,
       )));
     }
@@ -474,6 +474,9 @@ public function delete() {
     $this->data = array();
     $this->storage->delete($this->name);
     $this->isNew = TRUE;
+    $this->settingsOverrides = array();
+    $this->languageOverrides = array();
+    $this->moduleOverrides = array();
     $this->resetOverriddenData();
     $this->eventDispatcher->dispatch(ConfigEvents::DELETE, new ConfigCrudEvent($this));
     $this->originalData = $this->data;
diff --git a/core/tests/Drupal/Tests/Core/Config/ConfigTest.php b/core/tests/Drupal/Tests/Core/Config/ConfigTest.php
new file mode 100644
index 0000000..9272e60
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Config/ConfigTest.php
@@ -0,0 +1,337 @@
+<?php
+
+namespace Drupal\Tests\Core\Config;
+
+use Drupal\Tests\UnitTestCase;
+use Drupal\Core\Config\Config;
+use Drupal\Component\Utility\String;
+
+/**
+ * Tests the Config.
+ *
+ * @group Drupal
+ * @group Config
+ *
+ * @see \Drupal\Core\Config\Config
+ */
+class ConfigTest extends UnitTestCase {
+
+  /**
+   * Config.
+   *
+   * @var \Drupal\Core\Config\Config
+   */
+  protected $config;
+
+  /**
+   * Storage.
+   *
+   * @var \Drupal\Core\Config\StorageInterface
+   */
+  protected $storage;
+
+  /**
+   * Event Dispatcher.
+   *
+   * @var \Symfony\Component\EventDispatcher\EventDispatcherInterface
+   */
+  protected $event_dispatcher;
+
+  /**
+   * Typed Config.
+   *
+   * @var \Drupal\Core\Config\TypedConfigManagerInterface
+   */
+  protected $typed_config;
+
+  /**
+   * Test Info.
+   */
+  public static function getInfo() {
+    return array(
+      'name' => 'Config test',
+      'description' => 'Tests Config.',
+      'group' => 'Configuration'
+    );
+  }
+
+  /**
+   * Setup.
+   */
+  public function setUp() {
+    $this->storage = $this->getMock('Drupal\Core\Config\StorageInterface');
+    $this->event_dispatcher = $this->getMock('Symfony\Component\EventDispatcher\EventDispatcherInterface');
+    $this->typed_config = $this->getMock('\Drupal\Core\Config\TypedConfigManagerInterface');
+    $this->language = $this->getMock('\Drupal\Core\Language\Language');
+    $this->config = new Config('config.test', $this->storage, $this->event_dispatcher, $this->typed_config, $this->language);
+  }
+
+  /**
+   * Check that the config name is set correctly.
+   */
+  public function testSetName() {
+    // Valid name with dot.
+    $testName = 'test.name';
+
+    // Set the name.
+    $this->config->setName($testName);
+
+    // Check that the name has been set correctly.
+    $this->assertEquals($testName, $this->config->getName());
+
+    // Check that the name validates.
+    // Should throw \Drupal\Core\Config\ConfigNameException if invalid.
+    $this->config->validateName($testName);
+  }
+
+  /**
+   * Check that isNew is set correctly.
+   */
+  public function testIsNew() {
+    // Config should be new by default.
+    $this->assertTrue($this->config->isNew());
+
+    // Config is no longer new once saved.
+    $this->config->save();
+    $this->assertFalse($this->config->isNew());
+  }
+
+  /**
+   * Check that data is set correctly.
+   */
+  public function testSetData() {
+    $data = array('a' => 1, 'b' => 2, 'c' => array('d' => 3));
+    $this->config->setData($data);
+    $this->assertEquals($data, $this->config->getRawData());
+    $this->assertEquals(1, $this->config->get('a'));
+    $this->assertEquals(3, $this->config->get('c.d'));
+  }
+
+  /**
+   * Check that original data is set when config is saved.
+   */
+  public function testSave() {
+    // Set initial data.
+    $this->config->setData(array('a' => 'testVal'));
+
+    // Check that original data has not been set yet.
+    $this->assertNull($this->config->getOriginal('a', FALSE));
+
+    // Save so that the original data is set.
+    $config = $this->config->save();
+
+    // Check that returned $config is instance of Config.
+    $this->assertInstanceOf('\Drupal\Core\Config\Config', $config);
+
+    // Check that the original data it saved.
+    $this->assertEquals($this->config->getOriginal('a', FALSE), 'testVal');
+  }
+
+  /**
+   * Check that overrides are returned by get method and original data is maintained.
+   */
+  public function testOverrideData() {
+    // Set initial data.
+    $this->config->setData(array('a' => 1));
+
+    // Check original data was set correctly.
+    $this->assertEquals($this->config->get('a'), 1);
+
+    // Save so that the original data is stored.
+    $this->config->save();
+
+    // Set language override data and check value before and after save.
+    $this->config->setLanguageOverride(array('a' => 2));
+    $this->assertEquals($this->config->get('a'), 2);
+    $this->config->save();
+    $this->assertEquals($this->config->get('a'), 2);
+
+    // Set module override data and check value before and after save.
+    $this->config->setModuleOverride(array('a' => 3));
+    $this->assertEquals($this->config->get('a'), 3);
+    $this->config->save();
+    $this->assertEquals($this->config->get('a'), 3);
+
+    // Set settings override data and check value before and after save.
+    $this->config->setSettingsOverride(array('a' => 4));
+    $this->assertEquals($this->config->get('a'), 4);
+    $this->config->save();
+    $this->assertEquals($this->config->get('a'), 4);
+
+    // Set module and language again to ensure override order is correct.
+    $this->config->setLanguageOverride(array('a' => 2));
+    $this->config->setModuleOverride(array('a' => 3));
+
+    // 'a' should still be '4' after setting module and language overrides.
+    $this->assertEquals($this->config->get('a'), 4);
+    $this->config->save();
+    $this->assertEquals($this->config->get('a'), 4);
+
+    // Check original data has not changed.
+    $this->assertEquals($this->config->getOriginal('a', FALSE), 1);
+
+    // Check correct override value '4' is returned with $apply_overrides = TRUE.
+    $this->assertEquals($this->config->getOriginal('a', TRUE), 4);
+
+    // Check $apply_overrides defaults to TRUE.
+    $this->assertEquals($this->config->getOriginal('a'), 4);
+  }
+
+  /**
+   * Check that data is set correctly.
+   */
+  public function testSetValue() {
+    // Check single value.
+    $this->config->set('testData', 'testDataValue');
+    $this->assertEquals('testDataValue', $this->config->get('testData'));
+
+    // Check nested value.
+    $this->config->set('nested.testData', 'nestedDataValue');
+    $this->assertEquals('nestedDataValue', $this->config->get('nested.testData'));
+  }
+
+  /**
+   * Check that single value cannot be overwritten with a nested value.
+   *
+   * @expectedException PHPUnit_Framework_Error_Warning
+   */
+  public function testSetIllegalOffsetValue() {
+    // Set a single value.
+    $this->config->set('testData', 'testDataValue');
+
+    // Attempt to treat the single value as a nested item.
+    $this->config->set('testData.illegalOffset', 'testDataValue');
+  }
+
+  /**
+   * Check that config can be initialized with data.
+   */
+  public function testInitWithData() {
+    $config = $this->config->initWithData(array('a' => 'b'));
+
+    // Should return the Config object.
+    $this->assertInstanceOf('\Drupal\Core\Config\Config', $config);
+
+    // Check config is not new.
+    $this->assertEquals(FALSE, $this->config->isNew());
+
+    // Check that data value was set correctly.
+    $this->assertEquals('b', $this->config->get('a'));
+
+    // Check that original data was set.
+    $this->assertEquals('b', $this->config->getOriginal('a'));
+  }
+
+  /**
+   * Check clear.
+   */
+  public function testClear() {
+    // Check that value is cleared.
+    $this->config->set('a', 'testVal');
+    $this->assertEquals('testVal', $this->config->get('a'));
+    $this->config->clear('a');
+    $this->assertNull($this->config->get('a'));
+
+    // Check that nested value is cleared.
+    $this->config->set('a', array('b' => 'testVal'));
+    $this->assertEquals('testVal', $this->config->get('a.b'));
+    $this->config->clear('a.b');
+    $this->assertNull($this->config->get('a.b'));
+  }
+
+  /**
+   * Check that config delete is working correctly.
+   */
+  public function testDelete() {
+    // Set initial data.
+    $this->config->set('a', 'testVal');
+    $this->config->setModuleOverride(array('a' => 'overrideVal'));
+
+    // Save.
+    $this->config->save();
+
+    // Check that values have been set correctly.
+    $this->assertEquals('overrideVal', $this->config->get('a'));
+    $this->assertEquals('overrideVal', $this->config->getOriginal('a', TRUE));
+    $this->assertEquals('testVal', $this->config->getOriginal('a', FALSE));
+    $this->assertFalse($this->config->isNew());
+
+    // Delete.
+    $this->config->delete();
+
+    // Check object properties have been reset.
+    $this->assertTrue($this->config->isNew());
+    $this->assertEmpty($this->config->get('a'));
+    $this->assertEmpty($this->config->getOriginal('a', TRUE));
+    $this->assertEmpty($this->config->getOriginal('a', FALSE));
+  }
+
+  /**
+   * Check that data merges correctly.
+   */
+  public function testMerge() {
+    // Set initial data.
+    $data = array('a' => 1, 'b' => 2, 'c' => array('d' => 3));
+    $this->config->setData($data);
+
+    // Data to merge.
+    $dataToMerge = array('a' => 2, 'e' => 4, 'c' => array('f' => 5));
+    $this->config->merge($dataToMerge);
+
+    // Check that data has merged correctly.
+    $mergedData = array('a' => 2, 'b' => 2, 'c' => array('d' => 3, 'f' => 5), 'e' => 4);
+    $this->assertEquals($mergedData, $this->config->getRawData());
+  }
+
+  /**
+   * Check language is set.
+   */
+  public function testGetLanguage() {
+    $this->assertInstanceOf('\Drupal\Core\Language\Language', $this->config->getLanguage());
+  }
+
+  /**
+   * Checks that name validation exception are thrown.
+   *
+   * @expectedException \Drupal\Core\Config\ConfigNameException
+   * @dataProvider validateNameProvider
+   */
+  public function testValidateNameException($name, $exception_message) {
+    $this->setExpectedException('\Drupal\Core\Config\ConfigNameException', $exception_message);
+    $this->config->validateName($name);
+  }
+
+  /**
+   * Provides data to test name validation.
+   */
+  public function validateNameProvider() {
+    $return = array(
+      // Name missing namespace (dot).
+      array(
+        'MissingNamespace',
+        String::format('Missing namespace in Config object name MissingNamespace.', array(
+          '@name' => 'MissingNamespace',
+        )),
+      ),
+      // Exceeds length (max length plus an extra dot).
+      array(
+        str_repeat('a', Config::MAX_NAME_LENGTH) . ".",
+        String::format('Config object name @name exceeds maximum allowed length of @length characters.', array(
+          '@name' => str_repeat('a', Config::MAX_NAME_LENGTH) . ".",
+          '@length' => Config::MAX_NAME_LENGTH,
+        )),
+      ),
+    );
+    // Name must not contain : ? * < > " ' / \
+    foreach (array(':', '?', '*', '<', '>', '"',"'",'/','\\') as $char) {
+      $name = 'name.' . $char;
+      $return[] = array(
+        $name,
+        String::format('Invalid character in Config object name @name.', array(
+          '@name' => $name,
+        ))
+      );
+    }
+    return $return;
+  }
+}