diff --git a/src/JsonSchema/Constraints/Undefined.php b/src/JsonSchema/Constraints/Undefined.php
index 94264cca..c62e4465 100644
--- a/src/JsonSchema/Constraints/Undefined.php
+++ b/src/JsonSchema/Constraints/Undefined.php
@@ -9,6 +9,8 @@
 
 namespace JsonSchema\Constraints;
 
+use JsonSchema\Validator;
+
 /**
  * The Undefined Constraints
  *
@@ -89,6 +91,9 @@ protected function validateCommonProperties($value, $schema = null, $path = null
     {
         // if it extends another schema, it must pass that schema as well
         if (isset($schema->extends)) {
+            if (is_string($schema->extends)) {
+                $schema->extends = $this->validateUri($schema->extends, $schema, $path, $i);
+            }
             $this->checkUndefined($value, $schema->extends, $path, $i);
         }
 
@@ -115,4 +120,14 @@ protected function validateCommonProperties($value, $schema = null, $path = null
             }
         }
     }
+    
+    protected function validateUri($schemaUri = null, $schema, $path = null, $i = null)
+    {
+        $resolver = new \JsonSchema\Uri\UriResolver();
+        
+        if ($resolver->isValid($schemaUri)) {
+            $schemaId = property_exists($schema, 'id') ? $schema->id : null;
+            return Validator::retrieveUri($resolver->resolve($schemaUri, $schemaId));
+        }
+    }
 }
diff --git a/src/JsonSchema/Exception/InvalidSchemaMediaTypeException.php b/src/JsonSchema/Exception/InvalidSchemaMediaTypeException.php
new file mode 100644
index 00000000..2e946770
--- /dev/null
+++ b/src/JsonSchema/Exception/InvalidSchemaMediaTypeException.php
@@ -0,0 +1,17 @@
+<?php
+
+/*
+ * This file is part of the JsonSchema package.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace JsonSchema\Exception;
+
+/**
+ * Wrapper for the InvalidSchemaMediaType
+ */
+class InvalidSchemaMediaTypeException extends \RuntimeException
+{    
+}
\ No newline at end of file
diff --git a/src/JsonSchema/Exception/JsonDecodingException.php b/src/JsonSchema/Exception/JsonDecodingException.php
new file mode 100644
index 00000000..d27de6b2
--- /dev/null
+++ b/src/JsonSchema/Exception/JsonDecodingException.php
@@ -0,0 +1,40 @@
+<?php
+
+/*
+ * This file is part of the JsonSchema package.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace JsonSchema\Exception;
+
+/**
+ * Wrapper for the JsonDecodingException
+ */
+class JsonDecodingException extends \RuntimeException
+{
+    public function __construct($code = JSON_ERROR_NONE, \Exception $previous = null)
+    {
+        switch ($code) {
+            case JSON_ERROR_DEPTH:
+                $message = 'The maximum stack depth has been exceeded';
+                break;
+            case JSON_ERROR_STATE_MISMATCH:
+                $message = 'Invalid or malformed JSON';
+                break;
+            case JSON_ERROR_CTRL_CHAR:
+                $message = 'Control character error, possibly incorrectly encoded';
+                break;
+            case JSON_ERROR_UTF8:
+                $message = 'Malformed UTF-8 characters, possibly incorrectly encoded';
+                break;
+            case JSON_ERROR_SYNTAX:
+                $message = 'The maximum stack depth has been exceeded';
+                break;
+            default:
+                $message = 'Syntax error';
+        }
+        parent::__construct($message, $code, $previous);
+    }
+}
\ No newline at end of file
diff --git a/src/JsonSchema/Exception/ResourceNotFoundException.php b/src/JsonSchema/Exception/ResourceNotFoundException.php
new file mode 100644
index 00000000..87b4ac1b
--- /dev/null
+++ b/src/JsonSchema/Exception/ResourceNotFoundException.php
@@ -0,0 +1,17 @@
+<?php
+
+/*
+ * This file is part of the JsonSchema package.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace JsonSchema\Exception;
+
+/**
+ * Wrapper for the ResourceNotFoundException
+ */
+class ResourceNotFoundException extends \RuntimeException
+{
+}
\ No newline at end of file
diff --git a/src/JsonSchema/Exception/UriResolverException.php b/src/JsonSchema/Exception/UriResolverException.php
new file mode 100644
index 00000000..6d0b0949
--- /dev/null
+++ b/src/JsonSchema/Exception/UriResolverException.php
@@ -0,0 +1,17 @@
+<?php
+
+/*
+ * This file is part of the JsonSchema package.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace JsonSchema\Exception;
+
+/**
+ * Wrapper for the UriResolverException
+ */
+class UriResolverException extends \RuntimeException
+{
+}
\ No newline at end of file
diff --git a/src/JsonSchema/Uri/Retrievers/Curl.php b/src/JsonSchema/Uri/Retrievers/Curl.php
new file mode 100644
index 00000000..239714da
--- /dev/null
+++ b/src/JsonSchema/Uri/Retrievers/Curl.php
@@ -0,0 +1,82 @@
+<?php
+
+/*
+ * This file is part of the JsonSchema package.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace JsonSchema\Uri\Retrievers;
+
+use JsonSchema\Validator;
+
+use JsonSchema\Exception\ResourceNotFoundException;
+
+/**
+ * Tries to retrieve JSON schemas from a URI using cURL library
+ * 
+ * @author Sander Coolen <sander@jibber.nl> 
+ */
+class Curl implements UriRetrieverInterface
+{
+    protected $contentType;
+    protected $messageBody;
+    
+    public function __construct()
+    {
+        if (!function_exists('curl_init')) {
+            throw new \RuntimeException("cURL not installed");
+        }
+    }
+    
+    public function retrieve($uri)
+    {
+        $ch = curl_init();
+        
+        curl_setopt($ch, CURLOPT_URL, $uri);
+        curl_setopt($ch, CURLOPT_HEADER, true);
+        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
+        curl_setopt($ch, CURLOPT_HTTPHEADER, array('Accept: ' . Validator::SCHEMA_MEDIA_TYPE));
+        
+        $response = curl_exec($ch);
+        if (false === $response) {
+            throw new ResourceNotFoundException('JSON schema not found');
+        }
+        
+        $this->fetchMessageBody($response);
+        $this->fetchContentType($response);
+        
+        curl_close($ch);
+        
+        return $this->messageBody;
+    }
+    
+    /**
+     * @param string $response cURL HTTP response
+     */
+    private function fetchMessageBody($response)
+    {
+        preg_match("/(?:\r\n){2}(.*)$/ms", $response, $match);
+        $this->messageBody = $match[1];
+    }
+    
+    /**
+     * @param string $response cURL HTTP response
+     * @return boolean Whether the Content-Type header was found or not
+     */
+    protected function fetchContentType($response)
+    {
+        if (0 < preg_match("/Content-Type:(\V*)/ims", $response, $match)) {
+            $this->contentType = trim($match[1]);
+            
+            return true;
+        }
+        return false;
+    }
+    
+    public function getContentType()
+    {
+        return $this->contentType;
+    }
+}
\ No newline at end of file
diff --git a/src/JsonSchema/Uri/Retrievers/FileGetContents.php b/src/JsonSchema/Uri/Retrievers/FileGetContents.php
new file mode 100644
index 00000000..9c508718
--- /dev/null
+++ b/src/JsonSchema/Uri/Retrievers/FileGetContents.php
@@ -0,0 +1,73 @@
+<?php
+
+/*
+ * This file is part of the JsonSchema package.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace JsonSchema\Uri\Retrievers;
+
+use JsonSchema\Validator;
+
+/**
+ * Tries to retrieve JSON schemas from a URI using file_get_contents()
+ * 
+ * @author Sander Coolen <sander@jibber.nl> 
+ */
+class FileGetContents implements UriRetrieverInterface
+{
+    protected $contentType;
+    protected $messageBody;
+    
+    public function retrieve($uri)
+    {
+        $context = stream_context_create(array(
+            'http' => array(
+                'method' => 'GET',
+                'header' => "Accept: " . Validator::SCHEMA_MEDIA_TYPE
+            )));
+        
+        $response = file_get_contents($uri);
+        if (false === $response) {
+            throw new ResourceNotFoundException('JSON schema not found');
+        }
+        
+        $this->messageBody = $response;
+        $this->fetchContentType($http_response_header);
+        
+        return $this->messageBody;
+    }
+    
+    /**
+     * @param array $headers HTTP Response Headers
+     * @return boolean Whether the Content-Type header was found or not
+     */
+    private function fetchContentType(array $headers)
+    {
+        foreach ($headers as $header) {
+            if ($this->contentType = self::getContentTypeMatchInHeader($header)) {
+                return true;
+            }
+        }
+        
+        return false;
+    }
+    
+    /**
+     * @param string $header
+     * @return string|null
+     */
+    protected static function getContentTypeMatchInHeader($header)
+    {
+        if (0 < preg_match("/Content-Type:(\V*)/ims", $header, $match)) {
+            return trim($match[1]);
+        }
+    }
+
+    public function getContentType()
+    {
+        return $this->contentType;
+    }
+}
\ No newline at end of file
diff --git a/src/JsonSchema/Uri/Retrievers/UriRetrieverInterface.php b/src/JsonSchema/Uri/Retrievers/UriRetrieverInterface.php
new file mode 100644
index 00000000..44bfaf1b
--- /dev/null
+++ b/src/JsonSchema/Uri/Retrievers/UriRetrieverInterface.php
@@ -0,0 +1,22 @@
+<?php
+
+/*
+ * This file is part of the JsonSchema package.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace JsonSchema\Uri\Retrievers;
+
+/**
+ * Interface for URI retrievers
+ * 
+ * @author Sander Coolen <sander@jibber.nl> 
+ */
+interface UriRetrieverInterface
+{
+    public function retrieve($uri);
+    
+    public function getContentType();
+}
\ No newline at end of file
diff --git a/src/JsonSchema/Uri/UriResolver.php b/src/JsonSchema/Uri/UriResolver.php
new file mode 100644
index 00000000..85cc94b8
--- /dev/null
+++ b/src/JsonSchema/Uri/UriResolver.php
@@ -0,0 +1,150 @@
+<?php
+
+/*
+ * This file is part of the JsonSchema package.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace JsonSchema\Uri;
+
+use JsonSchema\Exception\UriResolverException;
+
+/**
+ * Resolves JSON Schema URIs
+ * 
+ * @author Sander Coolen <sander@jibber.nl> 
+ */
+class UriResolver
+{
+    /**
+     * Parses a URI into five main components
+     * 
+     * @param string $uri
+     * @return array 
+     */
+    public function parse($uri)
+    {
+        preg_match('|^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?|', $uri, $match);
+
+        $components = array();
+        if (5 < count($match)) {
+            $components =  array(
+                'scheme'    => $match[2],
+                'authority' => $match[4],
+                'path'      => $match[5]
+            );
+        } 
+        if (7 < count($match)) {
+            $components['query'] = $match[7];
+        }
+        if (9 < count($match)) {
+            $components['fragment'] = $match[9];
+        }
+        
+        return $components;
+    }
+    
+    /**
+     * Builds a URI based on n array with the main components
+     * 
+     * @param array $components
+     * @return string 
+     */
+    public function generate(array $components)
+    {
+        $uri = $components['scheme'] . '://' 
+             . $components['authority']
+             . $components['path'];
+        
+        if (array_key_exists('query', $components)) {
+            $uri .= $components['query'];
+        }
+        if (array_key_exists('fragment', $components)) {
+            $uri .= $components['fragment'];
+        }
+        
+        return $uri;
+    }
+    
+    /**
+     * Resolves a URI
+     * 
+     * @param string $uri Absolute or relative
+     * @param type $baseUri Optional base URI
+     * @return string
+     */
+    public function resolve($uri, $baseUri = null)
+    {
+        $components = $this->parse($uri);
+        $path = $components['path'];
+        
+        if ((array_key_exists('scheme', $components)) && ('http' === $components['scheme'])) {
+            return $uri;
+        }
+        $baseComponents = $this->parse($baseUri);
+        $basePath = $baseComponents['path'];
+        
+        $baseComponents['path'] = self::combineRelativePathWithBasePath($path, $basePath);
+        
+        return $this->generate($baseComponents);
+    }
+    
+    /**
+     * Tries to glue a relative path onto an absolute one
+     * 
+     * @param string $relativePath
+     * @param string $basePath
+     * @return string Merged path
+     * @throws UriResolverException 
+     */
+    private static function combineRelativePathWithBasePath($relativePath, $basePath)
+    {
+        $relativePath = self::normalizePath($relativePath);
+        $basePathSegments = self::getPathSegments($basePath);
+        
+        preg_match('|^/?(\.\./(?:\./)*)*|', $relativePath, $match);
+        $numLevelUp = strlen($match[0]) /3 + 1;
+        if ($numLevelUp >= count($basePathSegments)) {
+            throw new UriResolverException(sprintf("Unable to resolve URI '%s' from base '%s'", $relativePath, $basePath));
+        }
+        $basePathSegments = array_slice($basePathSegments, 0, -$numLevelUp);
+        $path = preg_replace('|^/?(\.\./(\./)*)*|', '', $relativePath);
+        
+        return implode(DIRECTORY_SEPARATOR, $basePathSegments) . '/' . $path;
+    }
+    
+    /**
+     * Normalizes a URI path component by removing dot-slash and double slashes
+     * 
+     * @param string $path
+     * @return string
+     */
+    private static function normalizePath($path)
+    {
+        $path = preg_replace('|((?<!\.)\./)*|', '', $path);
+        $path = preg_replace('|//|', '/', $path);
+        
+        return $path;
+    }
+    
+    /**
+     * @return array
+     */
+    private static function getPathSegments($path) {
+        
+        return explode(DIRECTORY_SEPARATOR, $path);
+    }
+    
+    /**
+     * @param string $uri
+     * @return boolean 
+     */
+    public function isValid($uri)
+    {
+        $components = $this->parse($uri);
+        
+        return !empty($components);
+    }
+}
\ No newline at end of file
diff --git a/src/JsonSchema/Validator.php b/src/JsonSchema/Validator.php
index db492125..c98ed229 100644
--- a/src/JsonSchema/Validator.php
+++ b/src/JsonSchema/Validator.php
@@ -12,6 +12,11 @@
 use JsonSchema\Constraints\Schema;
 use JsonSchema\Constraints\Constraint;
 
+use JsonSchema\Exception\InvalidSchemaMediaTypeException;
+use JsonSchema\Exception\JsonDecodingException;
+
+use JsonSchema\Uri\Retrievers\UriRetrieverInterface;
+
 /**
  * A JsonSchema Constraint
  *
@@ -21,6 +26,10 @@
  */
 class Validator extends Constraint
 {
+    const SCHEMA_MEDIA_TYPE = 'application/schema+json';
+    
+    private static $uriRetriever;
+
     /**
      * Validates the given data against the schema and returns an object containing the results
      * Both the php object and the schema are supposed to be a result of a json_decode call.
@@ -35,4 +44,38 @@ public function check($value, $schema = null, $path = null, $i = null)
 
         $this->addErrors($validator->getErrors());
     }
+    
+    /**
+     * Sets the URI retriever the validator will use. FileGetContents by default
+     * 
+     * @param UriRetrieverInterface $retriever
+     */
+    public static function setUriRetriever(UriRetrieverInterface $retriever)
+    {
+        self::$uriRetriever = $retriever;
+    }
+    
+    /**
+     * @param string $uri JSON Schema URI
+     * @return string JSON Schema contents
+     * @throws InvalidSchemaMediaType for invalid media types
+     */
+    public static function retrieveUri($uri)
+    {
+        if (null === self::$uriRetriever) {
+            self::setUriRetriever(new Uri\Retrievers\FileGetContents);
+        }
+        $contents = self::$uriRetriever->retrieve($uri);
+        if (self::SCHEMA_MEDIA_TYPE !== self::$uriRetriever->getContentType()) {
+            throw new InvalidSchemaMediaTypeException(sprintf('Media type %s expected', self::SCHEMA_MEDIA_TYPE));
+        }
+        $jsonSchema = json_decode($contents);
+        if (JSON_ERROR_NONE < $error = json_last_error()) {
+            throw new JsonDecodingException($error);
+        }
+        
+        // TODO validate using schema)
+        $jsonSchema->id = $uri;
+        return $jsonSchema;
+    }
 }
\ No newline at end of file
diff --git a/tests/JsonSchema/Tests/Uri/UriRetrieverTest.php b/tests/JsonSchema/Tests/Uri/UriRetrieverTest.php
new file mode 100644
index 00000000..acf58b69
--- /dev/null
+++ b/tests/JsonSchema/Tests/Uri/UriRetrieverTest.php
@@ -0,0 +1,153 @@
+<?php
+
+/*
+ * This file is part of the JsonSchema package.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace JsonSchema\Tests\Uri;
+
+use JsonSchema\Validator;
+
+class UriRetrieverTest extends \PHPUnit_Framework_TestCase
+{
+    protected $validator;
+
+    protected function setUp()
+    {
+        $this->validator = new Validator();
+    }
+    
+    private function getCurlRetrieverMock($returnSchema, $returnMediaType = Validator::SCHEMA_MEDIA_TYPE)
+    {
+        $curlRetriever = $this->getMock('JsonSchema\Uri\Retrievers\Curl', array('retrieve', 'getContentType'));
+        
+        $curlRetriever->expects($this->at(0))
+                      ->method('retrieve')
+                      ->with($this->equalTo('http://some.host.at/somewhere/parent'))
+                      ->will($this->returnValue($returnSchema));
+        
+        $curlRetriever->expects($this->atLeastOnce()) // index 1 and/or 3
+                      ->method('getContentType')
+                      ->will($this->returnValue($returnMediaType));
+        
+        return $curlRetriever;
+    }
+    
+    /**
+     * @dataProvider jsonProvider 
+     */
+    public function testChildExtendsParent($childSchema, $parentSchema)
+    {
+        $curlRetrieverMock = $this->getCurlRetrieverMock($parentSchema);
+        
+        Validator::setUriRetriever($curlRetrieverMock);
+        
+        $json = '{"childProp":"infant", "parentProp":false}';
+        $decodedJson = json_decode($json);
+        $decodedJsonSchema = json_decode($childSchema);
+        
+        $this->validator->check($decodedJson, $decodedJsonSchema);
+        $this->assertTrue($this->validator->isValid());
+    }
+    
+    /**
+     * @dataProvider jsonProvider 
+     */
+    public function testResolveRelativeUri($childSchema, $parentSchema)
+    {
+        self::setParentSchemaExtendsValue($parentSchema, 'grandparent');
+        $curlRetrieverMock = $this->getCurlRetrieverMock($parentSchema);
+        
+        $curlRetrieverMock->expects($this->at(2))
+                          ->method('retrieve')
+                          ->with($this->equalTo('http://some.host.at/somewhere/grandparent'))
+                          ->will($this->returnValue('{"type":"object","title":"grand-parent"}'));
+        
+        Validator::setUriRetriever($curlRetrieverMock);
+        
+        $json = '{"childProp":"infant", "parentProp":false}';
+        $decodedJson = json_decode($json);
+        $decodedJsonSchema = json_decode($childSchema);
+        
+        $this->validator->check($decodedJson, $decodedJsonSchema);
+        $this->assertTrue($this->validator->isValid());
+    }
+    
+    private static function setParentSchemaExtendsValue(&$parentSchema, $value)
+    {
+        $parentSchemaDecoded = json_decode($parentSchema, true);
+        $parentSchemaDecoded['extends'] = $value;
+        $parentSchema = json_encode($parentSchemaDecoded);
+    }
+    
+    /**
+     * @dataProvider jsonProvider
+     * @expectedException JsonSchema\Exception\InvalidSchemaMediaTypeException
+     */
+    public function testInvalidSchemaMediaType($childSchema, $parentSchema)
+    {
+        $curlRetrieverMock = $this->getCurlRetrieverMock($parentSchema, 'text/html');
+        
+        Validator::setUriRetriever($curlRetrieverMock);
+        
+        $json = '{"childProp":"infant", "parentProp":false}';
+        $decodedJson = json_decode($json);
+        $decodedJsonSchema = json_decode($childSchema);
+        
+        $this->validator->check($decodedJson, $decodedJsonSchema);
+    }
+    
+    /**
+     * @dataProvider jsonProvider
+     * @expectedException JsonSchema\Exception\JsonDecodingException
+     */
+    public function testParentJsonError($childSchema, $parentSchema)
+    {
+        $curlRetrieverMock = $this->getCurlRetrieverMock('<html>', 'application/schema+json');
+        
+        Validator::setUriRetriever($curlRetrieverMock);
+        
+        $json = '{}';
+        $decodedJson = json_decode($json);
+        $decodedJsonSchema = json_decode($childSchema);
+        
+        $this->validator->check($decodedJson, $decodedJsonSchema);
+    }
+    
+    public function jsonProvider()
+    {
+        $childSchema = <<<EOF
+{
+    "type":"object",    
+    "title":"child",
+    "extends":"http://some.host.at/somewhere/parent",
+    "properties":
+    {
+        "childProp":
+        {
+            "type":"string"
+        }
+    }
+}
+EOF;
+        $parentSchema = <<<EOF
+{
+    "type":"object",    
+    "title":"parent",
+    "properties":
+    {
+        "parentProp":
+        {
+            "type":"boolean"
+        }
+    }
+}
+EOF;
+        return array(
+            array($childSchema, $parentSchema)
+        );
+    }
+}