diff -u b/core/modules/hal/lib/Drupal/hal/Normalizer/EntityReferenceItemNormalizer.php b/core/modules/hal/lib/Drupal/hal/Normalizer/EntityReferenceItemNormalizer.php
--- b/core/modules/hal/lib/Drupal/hal/Normalizer/EntityReferenceItemNormalizer.php
+++ b/core/modules/hal/lib/Drupal/hal/Normalizer/EntityReferenceItemNormalizer.php
@@ -66,7 +66,10 @@
     $field_item = $context['target_instance'];
     $field_definition = $field_item->getDefinition();
     $target_type = $field_definition['settings']['target_type'];
-    return $this->entityResolver->resolve($this, $data, $target_type);
+    if ($id = $this->entityResolver->resolve($this, $data, $target_type)) {
+      return array('target_id' => $id);
+    }
+    return NULL;
   }
 
   /**
diff -u b/core/modules/serialization/lib/Drupal/serialization/EntityResolver/ChainEntityResolver.php b/core/modules/serialization/lib/Drupal/serialization/EntityResolver/ChainEntityResolver.php
--- b/core/modules/serialization/lib/Drupal/serialization/EntityResolver/ChainEntityResolver.php
+++ b/core/modules/serialization/lib/Drupal/serialization/EntityResolver/ChainEntityResolver.php
@@ -34,9 +34,9 @@
   /**
    * Implements \Drupal\serialization\EntityResolver\EntityResolverInterface::resolve().
    */
-  public function resolve(NormalizerInterface $normalizer, $data, $target_type) {
+  public function resolve(NormalizerInterface $normalizer, $data, $entity_type) {
     foreach ($this->resolvers as $resolver) {
-      if ($resolved = $resolver->resolve($normalizer, $data, $target_type)) {
+      if ($resolved = $resolver->resolve($normalizer, $data, $entity_type)) {
         return $resolved;
       }
     }
diff -u b/core/modules/serialization/lib/Drupal/serialization/EntityResolver/EntityResolverInterface.php b/core/modules/serialization/lib/Drupal/serialization/EntityResolver/EntityResolverInterface.php
--- b/core/modules/serialization/lib/Drupal/serialization/EntityResolver/EntityResolverInterface.php
+++ b/core/modules/serialization/lib/Drupal/serialization/EntityResolver/EntityResolverInterface.php
@@ -13,22 +13,31 @@
 
   /**
-   * Resolve an ID to the corresponding entity.
+   * Returns the local ID of an entity referenced by serialized data.
    *
-   * If the local id is found, the target_id property should be set in the
-   * returned array. Otherwise, if the EntityResolver knows how to dereference
-   * the value, the dereferencer should be set.
+   * Drupal entities are loaded by and internally referenced by a local ID.
+   * Because different websites can use the same local ID to refer to different
+   * entities (e.g., node "1" can be a different node on foo.com and bar.com, or
+   * on example.com and staging.example.com), it is generally unsuitable for use
+   * in hypermedia data exchanges. Instead, UUIDs, URIs, or other globally
+   * unique IDs are preferred.
+   *
+   * This function takes a $data array representing partially deserialized data
+   * for an entity reference, and resolves it to a local entity ID. For example,
+   * depending on the data specification being used, $data might contain a
+   * 'uuid' key, a 'uri' key, a 'href' key, or some other data identifying the
+   * entity, and it is up to the implementor of this interface to resolve that
+   * appropriately for the specification being used.
    *
    * @param \Symfony\Component\Serializer\Normalizer\NormalizerInterface $normalizer
    *   The Normalizer which is handling the data.
    * @param array $data
    *   The data passed into the calling Normalizer.
-   * @param string $target_type
-   *   The target type of the entity reference field.
+   * @param string $entity_type
+   *   The type of entity being resolved; e.g., 'node' or 'user'.
    *
-   * @return array|NULL
-   *   Returns an array containing the target_id, if found. Otherwise, return
-   *   NULL.
+   * @return string|NULL
+   *   Returns the local entity ID, if found. Otherwise, returns NULL.
    */
-  public function resolve(NormalizerInterface $normalizer, $data, $target_type);
+  public function resolve(NormalizerInterface $normalizer, $data, $entity_type);
 
 }
diff -u b/core/modules/serialization/lib/Drupal/serialization/EntityResolver/UuidResolver.php b/core/modules/serialization/lib/Drupal/serialization/EntityResolver/UuidResolver.php
--- b/core/modules/serialization/lib/Drupal/serialization/EntityResolver/UuidResolver.php
+++ b/core/modules/serialization/lib/Drupal/serialization/EntityResolver/UuidResolver.php
@@ -9,16 +9,21 @@
 
 use Symfony\Component\Serializer\Normalizer\NormalizerInterface;
 
+/**
+ * Resolves entities from data that contains an entity UUID.
+ */
 class UuidResolver implements EntityResolverInterface {
 
   /**
    * Implements \Drupal\serialization\EntityResolver\EntityResolverInterface::resolve().
    */
-  public function resolve(NormalizerInterface $normalizer, $data, $target_type) {
+  public function resolve(NormalizerInterface $normalizer, $data, $entity_type) {
+    // The normalizer is what knows the specification of the data being
+    // deserialized. If it can return a UUID from that data, and if there's an
+    // entity with that UUID, then return its ID.
     if (($normalizer instanceof UuidReferenceInterface) && $uuid = $normalizer->getUuid($data)) {
-      // If an entity with this UUID exists on the site, return the local id.
-      if ($entity = entity_load_by_uuid($target_type, $uuid)) {
-        return array('target_id' => $entity->id());
+      if ($entity = entity_load_by_uuid($entity_type, $uuid)) {
+        return $entity->id();
       }
     }
     return NULL;