diff --git a/src/JsonSchema/Constraints/Factory.php b/src/JsonSchema/Constraints/Factory.php
index 5eadb17e..d67fc952 100644
--- a/src/JsonSchema/Constraints/Factory.php
+++ b/src/JsonSchema/Constraints/Factory.php
@@ -11,6 +11,7 @@
 
 use JsonSchema\Exception\InvalidArgumentException;
 use JsonSchema\SchemaStorage;
+use JsonSchema\SchemaStorageInterface;
 use JsonSchema\Uri\UriRetriever;
 use JsonSchema\UriRetrieverInterface;
 
@@ -68,7 +69,7 @@ class Factory
      * @param int $checkMode
      */
     public function __construct(
-        SchemaStorage $schemaStorage = null,
+        SchemaStorageInterface $schemaStorage = null,
         UriRetrieverInterface $uriRetriever = null,
         $checkMode = Constraint::CHECK_MODE_NORMAL
     ) {
diff --git a/src/JsonSchema/SchemaStorage.php b/src/JsonSchema/SchemaStorage.php
index ff4b67d5..03d28d10 100644
--- a/src/JsonSchema/SchemaStorage.php
+++ b/src/JsonSchema/SchemaStorage.php
@@ -8,7 +8,7 @@
 use JsonSchema\Uri\UriResolver;
 use JsonSchema\Uri\UriRetriever;
 
-class SchemaStorage
+class SchemaStorage implements SchemaStorageInterface
 {
     protected $uriRetriever;
     protected $uriResolver;
@@ -39,8 +39,7 @@ public function getUriResolver()
     }
 
     /**
-     * @param string $id
-     * @param object $schema
+     * {@inheritdoc}
      */
     public function addSchema($id, $schema = null)
     {
@@ -58,8 +57,7 @@ public function addSchema($id, $schema = null)
     }
 
     /**
-     * @param string $id
-     * @return object
+     * {@inheritdoc}
      */
     public function getSchema($id)
     {
@@ -70,6 +68,9 @@ public function getSchema($id)
         return $this->schemas[$id];
     }
 
+    /**
+     * {@inheritdoc}
+     */
     public function resolveRef($ref)
     {
         $jsonPointer = new JsonPointer($ref);
@@ -93,8 +94,7 @@ public function resolveRef($ref)
     }
 
     /**
-     * @param $refSchema
-     * @return object
+     * {@inheritdoc}
      */
     public function resolveRefSchema($refSchema)
     {
diff --git a/src/JsonSchema/SchemaStorageInterface.php b/src/JsonSchema/SchemaStorageInterface.php
new file mode 100644
index 00000000..0f67478b
--- /dev/null
+++ b/src/JsonSchema/SchemaStorageInterface.php
@@ -0,0 +1,34 @@
+<?php
+
+namespace JsonSchema;
+
+interface SchemaStorageInterface
+{
+    /**
+     * Adds schema with given identifier
+     * @param string $id
+     * @param object $schema
+     */
+    public function addSchema($id, $schema = null);
+
+    /**
+     * Returns schema for given identifier, or null if it does not exist
+     * @param string $id
+     * @return object
+     */
+    public function getSchema($id);
+
+    /**
+     * Returns schema for given reference with all sub-references resolved
+     * @param string $ref
+     * @return object
+     */
+    public function resolveRef($ref);
+
+    /**
+     * Returns schema referenced by '$ref' property
+     * @param mixed $refSchema
+     * @return object
+     */
+    public function resolveRefSchema($refSchema);
+}