diff --git a/core/modules/comment/lib/Drupal/comment/CommentStorageController.php b/core/modules/comment/lib/Drupal/comment/CommentStorageController.php
index 8cd1a53..b990c58 100644
--- a/core/modules/comment/lib/Drupal/comment/CommentStorageController.php
+++ b/core/modules/comment/lib/Drupal/comment/CommentStorageController.php
@@ -137,6 +137,9 @@ public function baseFieldDefinitions() {
       'label' => t('Subject'),
       'description' => t('The comment title or subject.'),
       'type' => 'string_field',
+      'property_constraints' => array(
+        'value' => array('Length' => array('max' => 64)),
+      ),
     );
     $properties['uid'] = array(
       'label' => t('User ID'),
@@ -152,21 +155,36 @@ public function baseFieldDefinitions() {
       'description' => t("The comment author's name."),
       'type' => 'string_field',
       'settings' => array('default_value' => ''),
+      'property_constraints' => array(
+        'value' => array(
+          'Length' => array('max' => 60),
+          'CommentName' => array(),
+        ),
+      ),
     );
     $properties['mail'] = array(
       'label' => t('e-mail'),
       'description' => t("The comment author's e-mail address."),
+      // @todo Convert this to email_field when that data type actually works.
       'type' => 'string_field',
     );
     $properties['homepage'] = array(
       'label' => t('Homepage'),
       'description' => t("The comment author's home page address."),
-      'type' => 'string_field',
+      'type' => 'uri_field',
+      // URIs are not length limited by RFC 2616, but we can only store 255
+      // characters in our DB schema.
+      'property_constraints' => array(
+        'value' => array('Length' => array('max' => 255)),
+      ),
     );
     $properties['hostname'] = array(
       'label' => t('Hostname'),
       'description' => t("The comment author's hostname."),
       'type' => 'string_field',
+      'property_constraints' => array(
+        'value' => array('Length' => array('max' => 255)),
+      ),
     );
     $properties['created'] = array(
       'label' => t('Created'),
@@ -187,6 +205,9 @@ public function baseFieldDefinitions() {
       'label' => t('Thread place'),
       'description' => t("The alphadecimal representation of the comment's place in a thread, consisting of a base 36 string prefixed by an integer indicating its length."),
       'type' => 'string_field',
+      'property_constraints' => array(
+        'value' => array('Length' => array('max' => 255)),
+      ),
     );
     $properties['node_type'] = array(
       // @todo: The bundle property should be stored so it's queryable.
diff --git a/core/modules/comment/lib/Drupal/comment/Plugin/Validation/Constraint/CommentNameConstraint.php b/core/modules/comment/lib/Drupal/comment/Plugin/Validation/Constraint/CommentNameConstraint.php
new file mode 100644
index 0000000..9540b67
--- /dev/null
+++ b/core/modules/comment/lib/Drupal/comment/Plugin/Validation/Constraint/CommentNameConstraint.php
@@ -0,0 +1,25 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\comment\Plugin\Validation\Constraint\CommentNameConstraint.
+ */
+
+namespace Drupal\comment\Plugin\Validation\Constraint;
+
+use Drupal\Component\Annotation\Plugin;
+use Drupal\Core\Annotation\Translation;
+use Symfony\Component\Validator\Constraint;
+
+/**
+ * Supports validating comment author names.
+ *
+ * @Plugin(
+ *   id = "CommentName",
+ *   label = @Translation("Comment author name", context = "Validation")
+ * )
+ */
+class CommentNameConstraint extends Constraint {
+
+  public $message = 'The name you used belongs to a registered user.';
+}
diff --git a/core/modules/comment/lib/Drupal/comment/Plugin/Validation/Constraint/CommentNameConstraintValidator.php b/core/modules/comment/lib/Drupal/comment/Plugin/Validation/Constraint/CommentNameConstraintValidator.php
new file mode 100644
index 0000000..05db466
--- /dev/null
+++ b/core/modules/comment/lib/Drupal/comment/Plugin/Validation/Constraint/CommentNameConstraintValidator.php
@@ -0,0 +1,40 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\comment\Plugin\Validation\Constraint\CommentNameConstraintValidator.
+ */
+
+namespace Drupal\comment\Plugin\Validation\Constraint;
+
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+
+/**
+ * Validates the CommentName constraint.
+ */
+class CommentNameConstraintValidator extends ConstraintValidator {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validate($value, Constraint $constraint) {
+    if (!isset($value) || $value === '') {
+      return;
+    }
+    $comment = $this->context->getMetadata()->getTypedData()->getParent()->getParent();
+
+    if ($comment->uid->value === 0) {
+      $query = db_select('users', 'u');
+      $query->addField('u', 'uid', 'uid');
+      $taken = $query
+        ->condition('name', db_like($value), 'LIKE')
+        ->countQuery()
+        ->execute()
+        ->fetchField();
+      if ($taken) {
+        $this->context->addViolation($constraint->message);
+      }
+    }
+  }
+}
diff --git a/core/modules/comment/lib/Drupal/comment/Tests/CommentValidationTest.php b/core/modules/comment/lib/Drupal/comment/Tests/CommentValidationTest.php
new file mode 100644
index 0000000..9efdfd1
--- /dev/null
+++ b/core/modules/comment/lib/Drupal/comment/Tests/CommentValidationTest.php
@@ -0,0 +1,119 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\comment\Tests\CommentValidationTest.
+ */
+
+namespace Drupal\comment\Tests;
+
+use Drupal\simpletest\DrupalUnitTestBase;
+
+/**
+ * Tests comment validation constraints.
+ */
+class CommentValidationTest extends DrupalUnitTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = array('comment', 'node', 'entity', 'field', 'text', 'field_sql_storage', 'user', 'system');
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Comment Validation',
+      'description' => 'Tests the comment validation constraints.',
+      'group' => 'Comment',
+    );
+  }
+
+  public function setUp() {
+    parent::setUp();
+    $this->installSchema('node', 'node');
+    $this->installSchema('node', 'node_field_data');
+    $this->installSchema('node', 'node_field_revision');
+    $this->installSchema('comment', 'node_comment_statistics');
+    $this->installSchema('user', 'users');
+    $this->installSchema('user', 'users_roles');
+    $this->installSchema('system', 'sequences');
+    user_install();
+  }
+
+  /**
+   * Tests the comment validation constraints.
+   */
+  public function testValidation() {
+    $node = entity_create('node', array(
+      'type' => 'page',
+      'title' => 'test',
+    ));
+    $node->save();
+
+    $comment = entity_create('comment', array('nid' => $node->id()));
+    $violations = $comment->validate();
+    $this->assertEqual(count($violations), 0, 'No violations when validating a default comment.');
+
+    $comment->set('subject', $this->randomString(65));
+    $this->assertLengthViolation($comment, 'subject', 64);
+    // Make the subject valid.
+    $comment->set('subject', $this->randomString());
+
+    $comment->set('name', $this->randomString(61));
+    $this->assertLengthViolation($comment, 'name', 60);
+    // Validate a name collision between an anonymous comment author name and an
+    // existing user account name.
+    $user = entity_create('user', array('name' => 'test'));
+    $user->save();
+    $comment->set('name', 'test');
+    $violations = $comment->validate();
+    $this->assertEqual(count($violations), 1, "Violation found on author name collision");
+    $this->assertEqual($violations[0]->getPropertyPath(), "name.0.value");
+    $this->assertEqual($violations[0]->getMessage(), t('The name you used belongs to a registered user.'));
+
+    // Make the name valid.
+    $comment->set('name', $this->randomString());
+
+    // @todo Enable this test once the conversion to email_field is done and
+    // works.
+    /*$comment->set('mail', 'invalid');
+    $violations = $comment->validate();
+    $this->assertEqual(count($violations), 1, 'Violation found when email is invalid');
+    $this->assertEqual($violations[0]->getPropertyPath(), 'mail.0.value');*/
+
+    $comment->set('homepage', 'http://example.com/' . $this->randomName(237));
+    $this->assertLengthViolation($comment, 'homepage', 255);
+    $comment->set('homepage', NULL);
+
+    // @todo add a test for invalid homepage URIs once the uri_field value is
+    // converted to uri, see https://drupal.org/node/2039961
+
+    $comment->set('hostname', $this->randomString(256));
+    $this->assertLengthViolation($comment, 'hostname', 255);
+    $comment->set('hostname', NULL);
+
+    $comment->set('thread', $this->randomString(256));
+    $this->assertLengthViolation($comment, 'thread', 255);
+
+    // @todo Add a test for the allowed values of CommentNewValue once
+    // https://drupal.org/node/1758622 is committed.
+  }
+
+  /**
+   * Verifies that a length violation exists for the given field.
+   *
+   * @param \Drupal\Core\Entity\ContentEntityInterface\CommentInterface $comment
+   *   The comment object to validate.
+   * @param string $field_name
+   *   The field that violates the maximum length.
+   * @param int $length
+   *   Number of characters that was exceeded.
+   */
+  protected function assertLengthViolation($comment, $field_name, $length) {
+    $violations = $comment->validate();
+    $this->assertEqual(count($violations), 1, "Violation found when $field_name is too long.");
+    $this->assertEqual($violations[0]->getPropertyPath(), "$field_name.0.value");
+    $this->assertEqual($violations[0]->getMessage(), t('This value is too long. It should have %limit characters or less.', array('%limit' => $length)));
+  }
+}
diff --git a/core/modules/node/lib/Drupal/node/NodeStorageController.php b/core/modules/node/lib/Drupal/node/NodeStorageController.php
index c3ccc3c..5efde56 100644
--- a/core/modules/node/lib/Drupal/node/NodeStorageController.php
+++ b/core/modules/node/lib/Drupal/node/NodeStorageController.php
@@ -160,7 +160,10 @@ public function baseFieldDefinitions() {
       'label' => t('User ID'),
       'description' => t('The user ID of the node author.'),
       'type' => 'entity_reference_field',
-      'settings' => array('target_type' => 'user'),
+      'settings' => array(
+        'target_type' => 'user',
+        'default_value' => 0,
+      ),
     );
     $properties['status'] = array(
       'label' => t('Publishing status'),