diff --git a/core/modules/comment/comment.install b/core/modules/comment/comment.install
index 618a091..0a147b5 100644
--- a/core/modules/comment/comment.install
+++ b/core/modules/comment/comment.install
@@ -92,11 +92,10 @@ function comment_schema() {
         'description' => 'Primary Key: Unique comment ID.',
       ),
       'uuid' => array(
-        'description' => 'The universally unique identifier for the comment.',
+        'description' => 'Unique Key: Universally unique identifier for this entity.',
         'type' => 'varchar',
         'length' => 128,
-        'not null' => TRUE,
-        'default' => '',
+        'not null' => FALSE,
       ),
       'pid' => array(
         'type' => 'int',
@@ -188,9 +187,11 @@ function comment_schema() {
       'comment_uid' => array('uid'),
       'comment_nid_langcode' => array('nid', 'langcode'),
       'comment_created' => array('created'),
-      'uuid' => array('uuid'),
     ),
     'primary key' => array('cid'),
+    'unique keys' => array(
+      'uuid' => array('uuid'),
+    ),
     'foreign keys' => array(
       'comment_node' => array(
         'table' => 'node',
@@ -296,14 +297,13 @@ function comment_update_8000() {
  */
 function comment_update_8001() {
   $spec = array(
-    'description' => 'The universally unique identifier for the comment.',
+    'description' => 'Unique Key: Universally unique identifier for this entity.',
     'type' => 'varchar',
     'length' => 128,
-    'not null' => TRUE,
-    'default' => '',
+    'not null' => FALSE,
   );
   $keys = array(
-    'indexes' => array(
+    'unique keys' => array(
       'uuid' => array('uuid'),
     ),
   );
diff --git a/core/modules/entity/lib/Drupal/entity/Tests/EntityUUIDTest.php b/core/modules/entity/lib/Drupal/entity/Tests/EntityUUIDTest.php
new file mode 100644
index 0000000..c2f42c5
--- /dev/null
+++ b/core/modules/entity/lib/Drupal/entity/Tests/EntityUUIDTest.php
@@ -0,0 +1,65 @@
+<?php
+
+/**
+ * @file
+ * Definition of Drupal\entity\Tests\EntityUUIDTest.
+ */
+
+namespace Drupal\entity\Tests;
+
+use Drupal\Component\Uuid\Uuid;
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests creation, saving, and loading of entity UUIDs.
+ */
+class EntityUUIDTest extends WebTestBase {
+  public static function getInfo() {
+    return array(
+      'name' => 'Entity UUIDs',
+      'description' => 'Tests creation, saving, and loading of entity UUIDs.',
+      'group' => 'Entity API',
+    );
+  }
+
+  function setUp() {
+    parent::setUp(array('entity_test'));
+  }
+
+  /**
+   * Tests UUID generation in entity CRUD operations.
+   */
+  function testCRUD() {
+    // Verify that no UUID is auto-generated when passing one for creation.
+    $uuid_service = new Uuid();
+    $uuid = $uuid_service->generate();
+    $custom_entity = entity_create('entity_test', array(
+      'name' => $this->randomName(),
+      'uuid' => $uuid,
+    ));
+    $this->assertIdentical($custom_entity->get('uuid'), $uuid);
+    // Save this entity, so we have more than one later.
+    $custom_entity->save();
+
+    // Verify that a new UUID is generated upon creating an entity.
+    $entity = entity_create('entity_test', array('name' => $this->randomName()));
+    $uuid = $entity->get('uuid');
+    $this->assertTrue($uuid);
+
+    // Verify that the new UUID is different.
+    $this->assertNotEqual($custom_entity->get('uuid'), $uuid);
+
+    // Verify that the UUID is retained upon saving.
+    $entity->save();
+    $this->assertIdentical($entity->get('uuid'), $uuid);
+
+    // Verify that the UUID is retained upon loading.
+    $entity_loaded = entity_test_load($entity->id(), TRUE);
+    $this->assertIdentical($entity_loaded->get('uuid'), $uuid);
+
+    // Verify that entity_load_by_uuid() loads the same entity.
+    $entity_loaded_by_uuid = entity_load_by_uuid('entity_test', $uuid, TRUE);
+    $this->assertIdentical($entity_loaded_by_uuid->get('uuid'), $uuid);
+    $this->assertEqual($entity_loaded_by_uuid, $entity_loaded);
+  }
+}
diff --git a/core/modules/entity/lib/Drupal/entity/Tests/EntityUuidTest.php b/core/modules/entity/lib/Drupal/entity/Tests/EntityUuidTest.php
deleted file mode 100644
index f416e05..0000000
--- a/core/modules/entity/lib/Drupal/entity/Tests/EntityUuidTest.php
+++ /dev/null
@@ -1,74 +0,0 @@
-<?php
-
-/**
- * @file
- * Definition of Drupal\entity\Tests\EntityUuidTest.
- */
-
-namespace Drupal\entity\Tests;
-
-use Drupal\simpletest\WebTestBase;
-
-/**
- * Tests related to node types.
- */
-class EntityUuidTest extends WebTestBase {
-
-  public static function getInfo() {
-    return array(
-      'name' => 'Entity UUID',
-      'description' => 'Ensures that UUIDs are saved and loaded successfully on UUID-capable entities.',
-      'group' => 'Entity API',
-    );
-  }
-
-  function setUp() {
-    parent::setUp('entity_test');
-  }
-
-  /**
-   * Test UUID generation on entities.
-   */
-  function testEntityUuidCreation() {
-    $entity = entity_create('entity_test', array('name' => 'test'));
-    $entity->save();
-    $this->assertTrue(strlen($entity->get('uuid')) == 36, 'Check that UUID length is 36');
-  }
-
-  /**
-   * Test saving and loading nodes with UUIDs.
-   */
-  function testEntityUuidNode() {
-    // Create two nodes.
-    $node1 = $this->drupalCreateNode();
-    $node2 = $this->drupalCreateNode();
-
-    // Make sure node saves are generating unique UUIDs.
-    $this->assertNotEqual($node1->get('uuid'), $node2->get('uuid'), 'Check that UUIDs are different');
-
-    // Check to make sure UUID did not get truncated when saving to {node}.
-    $this->assertTrue(strlen($node1->get('uuid')) == 36, 'Check that UUID length is 36');
-
-    // Check for UUID preservation on an entity resave.
-    $node1_reloaded = node_load($node1->nid);
-    node_save($node1_reloaded);
-    $this->assertEqual($node1->get('uuid'), $node1_reloaded->get('uuid'), 'Check that UUID did not change on node save.');
-  }
-
-  /**
-   * Test loading entities by UUIDs.
-   */
-  function testEntityLoadByUuid() {
-    $entity1 = entity_create('entity_test', array('name' => 'test1'));
-    $entity1->save();
-
-    $entity_loaded = entity_load_by_uuid('entity_test', $entity1->get('uuid'));
-
-    $this->assertEqual($entity1->id(), $entity_loaded->id(), 'Check that load by UUID loaded the same entity successfully.');
-
-    $entity1->save();
-
-    $this->assertEqual($entity1->get('uuid'), $entity_loaded->get('uuid'), 'Check that UUID did not change on entity save.');
-  }
-
-}
diff --git a/core/modules/entity/tests/modules/entity_test/entity_test.install b/core/modules/entity/tests/modules/entity_test/entity_test.install
index fb0711d..1bef390 100644
--- a/core/modules/entity/tests/modules/entity_test/entity_test.install
+++ b/core/modules/entity/tests/modules/entity_test/entity_test.install
@@ -44,11 +44,10 @@ function entity_test_schema() {
         'description' => 'Primary Key: Unique entity-test item ID.',
       ),
       'uuid' => array(
-        'description' => 'The universally unique identifier for the entity.',
+        'description' => 'Unique Key: Universally unique identifier for this entity.',
         'type' => 'varchar',
         'length' => 128,
-        'not null' => TRUE,
-        'default' => '',
+        'not null' => FALSE,
       ),
       'name' => array(
         'description' => 'The name of the test entity.',
@@ -74,12 +73,14 @@ function entity_test_schema() {
     ),
     'indexes' => array(
       'uid' => array('uid'),
-      'uuid' => array('uuid'),
     ),
     'foreign keys' => array(
       'uid' => array('users' => 'uid'),
     ),
     'primary key' => array('id'),
+    'unique keys' => array(
+      'uuid' => array('uuid'),
+    ),
   );
   return $schema;
 }
diff --git a/core/modules/node/node.install b/core/modules/node/node.install
index 9c0d4ca..054991d 100644
--- a/core/modules/node/node.install
+++ b/core/modules/node/node.install
@@ -18,6 +18,12 @@ function node_schema() {
         'unsigned' => TRUE,
         'not null' => TRUE,
       ),
+      'uuid' => array(
+        'description' => 'Unique Key: Universally unique identifier for this entity.',
+        'type' => 'varchar',
+        'length' => 128,
+        'not null' => FALSE,
+      ),
       // Defaults to NULL in order to avoid a brief period of potential
       // deadlocks on the index.
       'vid' => array(
@@ -27,13 +33,6 @@ function node_schema() {
         'not null' => FALSE,
         'default' => NULL,
       ),
-      'uuid' => array(
-        'description' => 'The universally unique identifier for the node.',
-        'type' => 'varchar',
-        'length' => 128,
-        'not null' => TRUE,
-        'default' => '',
-      ),
       'type' => array(
         'description' => 'The {node_type}.type of this node.',
         'type' => 'varchar',
@@ -121,10 +120,10 @@ function node_schema() {
       'uid'                 => array('uid'),
       'tnid'                => array('tnid'),
       'translate'           => array('translate'),
-      'uuid'                => array('uuid'),
     ),
     'unique keys' => array(
       'vid' => array('vid'),
+      'uuid' => array('uuid'),
     ),
     'foreign keys' => array(
       'node_revision' => array(
@@ -582,14 +581,13 @@ function node_update_8003() {
  */
 function node_update_8004() {
   $spec = array(
-    'description' => 'The universally unique identifier for the node.',
+    'description' => 'Unique Key: Universally unique identifier for this entity.',
     'type' => 'varchar',
     'length' => 128,
-    'not null' => TRUE,
-    'default' => '',
+    'not null' => FALSE,
   );
   $keys = array(
-    'indexes' => array(
+    'unique keys' => array(
       'uuid' => array('uuid'),
     ),
   );
diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index 5706b54..08d97c8 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -867,11 +867,10 @@ function system_schema() {
         'not null' => TRUE,
       ),
       'uuid' => array(
-        'description' => 'The universally unique identifier for the file.',
+        'description' => 'Unique Key: Universally unique identifier for this entity.',
         'type' => 'varchar',
         'length' => 128,
-        'not null' => TRUE,
-        'default' => '',
+        'not null' => FALSE,
       ),
       'uid' => array(
         'description' => 'The {users}.uid of the user who is associated with the file.',
@@ -936,9 +935,9 @@ function system_schema() {
       'uid' => array('uid'),
       'status' => array('status'),
       'timestamp' => array('timestamp'),
-      'uuid' => array('uuid'),
     ),
     'unique keys' => array(
+      'uuid' => array('uuid'),
       'uri' => array('uri'),
     ),
     'primary key' => array('fid'),
@@ -1952,14 +1951,13 @@ function system_update_8011() {
  */
 function system_update_8012() {
   $spec = array(
-    'description' => 'The universally unique identifier for the file.',
+    'description' => 'Unique Key: Universally unique identifier for this entity.',
     'type' => 'varchar',
     'length' => 128,
-    'not null' => TRUE,
-    'default' => '',
+    'not null' => FALSE,
   );
   $keys = array(
-    'indexes' => array(
+    'unique keys' => array(
       'uuid' => array('uuid'),
     ),
   );
diff --git a/core/modules/taxonomy/taxonomy.install b/core/modules/taxonomy/taxonomy.install
index d296036..361dc6a 100644
--- a/core/modules/taxonomy/taxonomy.install
+++ b/core/modules/taxonomy/taxonomy.install
@@ -33,11 +33,10 @@ function taxonomy_schema() {
         'description' => 'Primary Key: Unique term ID.',
       ),
       'uuid' => array(
-        'description' => 'The universally unique identifier for the term.',
+        'description' => 'Unique Key: Universally unique identifier for this entity.',
         'type' => 'varchar',
         'length' => 128,
-        'not null' => TRUE,
-        'default' => '',
+        'not null' => FALSE,
       ),
       'vid' => array(
         'type' => 'int',
@@ -82,6 +81,9 @@ function taxonomy_schema() {
       ),
     ),
     'primary key' => array('tid'),
+    'unique keys' => array(
+      'uuid' => array('uuid'),
+    ),
     'foreign keys' => array(
       'vocabulary' => array(
         'table' => 'taxonomy_vocabulary',
@@ -92,7 +94,6 @@ function taxonomy_schema() {
       'taxonomy_tree' => array('vid', 'weight', 'name'),
       'vid_name' => array('vid', 'name'),
       'name' => array('name'),
-      'uuid' => array('uuid'),
     ),
   );
 
@@ -316,14 +317,13 @@ function taxonomy_update_8001() {
  */
 function taxonomy_update_8002() {
   $spec = array(
-    'description' => 'The universally unique identifier for the term.',
+    'description' => 'Unique Key: Universally unique identifier for this entity.',
     'type' => 'varchar',
     'length' => 128,
-    'not null' => TRUE,
-    'default' => '',
+    'not null' => FALSE,
   );
   $keys = array(
-    'indexes' => array(
+    'unique keys' => array(
       'uuid' => array('uuid'),
     ),
   );
diff --git a/core/modules/user/user.install b/core/modules/user/user.install
index 2de1b24..abb985b 100644
--- a/core/modules/user/user.install
+++ b/core/modules/user/user.install
@@ -133,11 +133,10 @@ function user_schema() {
         'default' => 0,
       ),
       'uuid' => array(
-        'description' => 'The universally unique identifier for the user.',
+        'description' => 'Unique Key: Universally unique identifier for this entity.',
         'type' => 'varchar',
         'length' => 128,
-        'not null' => TRUE,
-        'default' => '',
+        'not null' => FALSE,
       ),
       'name' => array(
         'type' => 'varchar',
@@ -251,9 +250,9 @@ function user_schema() {
       'created' => array('created'),
       'mail' => array('mail'),
       'picture' => array('picture'),
-      'uuid' => array('uuid'),
     ),
     'unique keys' => array(
+      'uuid' => array('uuid'),
       'name' => array('name'),
     ),
     'primary key' => array('uid'),
@@ -456,14 +455,13 @@ function user_update_8002() {
  */
 function user_update_8003() {
   $spec = array(
-    'description' => 'The universally unique identifier for the user.',
+    'description' => 'Unique Key: Universally unique identifier for this entity.',
     'type' => 'varchar',
     'length' => 128,
-    'not null' => TRUE,
-    'default' => '',
+    'not null' => FALSE,
   );
   $keys = array(
-    'indexes' => array(
+    'unique keys' => array(
       'uuid' => array('uuid'),
     ),
   );