diff --git a/src/DataFormatter.php b/src/DataFormatter.php
index 2e2ace6..b093a08 100644
--- a/src/DataFormatter.php
+++ b/src/DataFormatter.php
@@ -260,6 +260,9 @@ abstract class DataFormatter
         return $this->removeFields;
     }
 
+    /**
+     * @return string
+     */
     public function getOutputContentType()
     {
         return $this->outputContentType;
@@ -340,15 +343,29 @@ abstract class DataFormatter
     abstract public function supportedMimeTypes();
 
     /**
-     * Convert a single data object to this format.  Return a string.
+     * Convert a single data object to this format. Return a string.
+     *
+     * @param DataObjectInterface $do
+     * @return mixed
      */
     abstract public function convertDataObject(DataObjectInterface $do);
 
     /**
-     * Convert a data object set to this format.  Return a string.
+     * Convert a data object set to this format. Return a string.
+     *
+     * @param SS_List $set
+     * @return string
      */
     abstract public function convertDataObjectSet(SS_List $set);
 
+    /**
+     * Convert an array to this format. Return a string.
+     *
+     * @param $array
+     * @return string
+     */
+    abstract public function convertArray($array);
+
     /**
      * @param string $strData HTTP Payload as string
      */
diff --git a/src/DataFormatter/JSONDataFormatter.php b/src/DataFormatter/JSONDataFormatter.php
index 173af0a..bcc643b 100644
--- a/src/DataFormatter/JSONDataFormatter.php
+++ b/src/DataFormatter/JSONDataFormatter.php
@@ -22,6 +22,9 @@ class JSONDataFormatter extends DataFormatter
 
     protected $outputContentType = 'application/json';
 
+    /**
+     * @return array
+     */
     public function supportedExtensions()
     {
         return array(
@@ -30,6 +33,9 @@ class JSONDataFormatter extends DataFormatter
         );
     }
 
+    /**
+     * @return array
+     */
     public function supportedMimeTypes()
     {
         return array(
@@ -38,6 +44,15 @@ class JSONDataFormatter extends DataFormatter
         );
     }
 
+    /**
+     * @param $array
+     * @return string
+     */
+    public function convertArray($array)
+    {
+        return Convert::array2json($array);
+    }
+
     /**
      * Generate a JSON representation of the given {@link DataObject}.
      *
@@ -163,6 +178,10 @@ class JSONDataFormatter extends DataFormatter
         return Convert::array2json($serobj);
     }
 
+    /**
+     * @param string $strData
+     * @return array|bool|void
+     */
     public function convertStringToArray($strData)
     {
         return Convert::json2array($strData);
diff --git a/src/DataFormatter/XMLDataFormatter.php b/src/DataFormatter/XMLDataFormatter.php
index 3876ea7..f2b9673 100644
--- a/src/DataFormatter/XMLDataFormatter.php
+++ b/src/DataFormatter/XMLDataFormatter.php
@@ -4,6 +4,7 @@ namespace SilverStripe\RestfulServer\DataFormatter;
 
 use SilverStripe\Control\Controller;
 use SilverStripe\Core\Convert;
+use SilverStripe\Dev\Debug;
 use SilverStripe\RestfulServer\DataFormatter;
 use SilverStripe\ORM\DataObject;
 use SilverStripe\ORM\DataObjectInterface;
@@ -24,6 +25,9 @@ class XMLDataFormatter extends DataFormatter
 
     protected $outputContentType = 'text/xml';
 
+    /**
+     * @return array
+     */
     public function supportedExtensions()
     {
         return array(
@@ -31,6 +35,9 @@ class XMLDataFormatter extends DataFormatter
         );
     }
 
+    /**
+     * @return array
+     */
     public function supportedMimeTypes()
     {
         return array(
@@ -39,6 +46,48 @@ class XMLDataFormatter extends DataFormatter
         );
     }
 
+    /**
+     * @param $array
+     * @return string
+     * @throws \Exception
+     */
+    public function convertArray($array)
+    {
+        $response = Controller::curr()->getResponse();
+        if ($response) {
+            $response->addHeader("Content-Type", "text/xml");
+        }
+
+        return "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n
+            <response>{$this->convertArrayWithoutHeader($array)}</response>";
+    }
+
+    /**
+     * @param $array
+     * @return string
+     * @throws \Exception
+     */
+    public function convertArrayWithoutHeader($array)
+    {
+        $xml = '';
+
+        foreach ($array as $fieldName => $fieldValue) {
+            if (is_array($fieldValue)) {
+                if (is_numeric($fieldName)) {
+                    $fieldName = 'Item';
+                }
+
+                $xml .= "<{$fieldName}>\n";
+                $xml .= $this->convertArrayWithoutHeader($fieldValue);
+                $xml .= "</{$fieldName}>\n";
+            } else {
+                $xml .= "<$fieldName>$fieldValue</$fieldName>\n";
+            }
+        }
+
+        return $xml;
+    }
+
     /**
      * Generate an XML representation of the given {@link DataObject}.
      *
@@ -56,6 +105,12 @@ class XMLDataFormatter extends DataFormatter
         return "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n" . $this->convertDataObjectWithoutHeader($obj, $fields);
     }
 
+    /**
+     * @param DataObject $obj
+     * @param null $fields
+     * @param null $relations
+     * @return string
+     */
     public function convertDataObjectWithoutHeader(DataObject $obj, $fields = null, $relations = null)
     {
         $className = $this->sanitiseClassName(get_class($obj));
@@ -195,6 +250,11 @@ class XMLDataFormatter extends DataFormatter
         return $xml;
     }
 
+    /**
+     * @param string $strData
+     * @return array|void
+     * @throws \Exception
+     */
     public function convertStringToArray($strData)
     {
         return Convert::xml2array($strData);
diff --git a/src/RestfulServer.php b/src/RestfulServer.php
index a245a03..d5791a3 100644
--- a/src/RestfulServer.php
+++ b/src/RestfulServer.php
@@ -10,6 +10,8 @@ use SilverStripe\ORM\DataObject;
 use SilverStripe\Control\Director;
 use SilverStripe\Control\HTTPRequest;
 use SilverStripe\ORM\SS_List;
+use SilverStripe\ORM\ValidationException;
+use SilverStripe\ORM\ValidationResult;
 use SilverStripe\Security\Member;
 use SilverStripe\Security\Security;
 use SilverStripe\CMS\Model\SiteTree;
@@ -443,8 +445,13 @@ class RestfulServer extends Controller
             return $this->unsupportedMediaType();
         }
 
-        /** @var DataObject|string */
-        $obj = $this->updateDataObject($obj, $reqFormatter);
+        try {
+            /** @var DataObject|string */
+            $obj = $this->updateDataObject($obj, $reqFormatter);
+        } catch (ValidationException $e) {
+            return $this->validationFailure($responseFormatter, $e->getResult());
+        }
+
         if (is_string($obj)) {
             return $obj;
         }
@@ -515,8 +522,13 @@ class RestfulServer extends Controller
 
         $responseFormatter = $this->getResponseDataFormatter($className);
 
-        /** @var DataObject|string $obj */
-        $obj = $this->updateDataObject($obj, $reqFormatter);
+        try {
+            /** @var DataObject|string $obj */
+            $obj = $this->updateDataObject($obj, $reqFormatter);
+        } catch (ValidationException $e) {
+            return $this->validationFailure($responseFormatter, $e->getResult());
+        }
+
         if (is_string($obj)) {
             return $obj;
         }
@@ -643,6 +655,9 @@ class RestfulServer extends Controller
         }
     }
 
+    /**
+     * @return string
+     */
     protected function permissionFailure()
     {
         // return a 401
@@ -652,6 +667,9 @@ class RestfulServer extends Controller
         return "You don't have access to this item through the API.";
     }
 
+    /**
+     * @return string
+     */
     protected function notFound()
     {
         // return a 404
@@ -660,6 +678,9 @@ class RestfulServer extends Controller
         return "That object wasn't found";
     }
 
+    /**
+     * @return string
+     */
     protected function methodNotAllowed()
     {
         $this->getResponse()->setStatusCode(405);
@@ -667,6 +688,9 @@ class RestfulServer extends Controller
         return "Method Not Allowed";
     }
 
+    /**
+     * @return string
+     */
     protected function unsupportedMediaType()
     {
         $this->response->setStatusCode(415); // Unsupported Media Type
@@ -674,6 +698,23 @@ class RestfulServer extends Controller
         return "Unsupported Media Type";
     }
 
+    /**
+     * @param ValidationResult $result
+     * @return mixed
+     */
+    protected function validationFailure(DataFormatter $responseFormatter, ValidationResult $result)
+    {
+        $this->getResponse()->setStatusCode(400);
+        $this->getResponse()->addHeader('Content-Type', $responseFormatter->getOutputContentType());
+
+        $response = [
+            'type' => ValidationException::class,
+            'messages' => $result->getMessages(),
+        ];
+
+        return $responseFormatter->convertArray($response);
+    }
+
     /**
      * A function to authenticate a user
      *
diff --git a/tests/unit/RestfulServerTest.php b/tests/unit/RestfulServerTest.php
index 39463ca..22bf37c 100644
--- a/tests/unit/RestfulServerTest.php
+++ b/tests/unit/RestfulServerTest.php
@@ -10,6 +10,7 @@ use SilverStripe\RestfulServer\Tests\Stubs\RestfulServerTestAuthorRating;
 use SilverStripe\Control\Director;
 use SilverStripe\Core\Convert;
 use SilverStripe\Control\Controller;
+use SilverStripe\RestfulServer\Tests\Stubs\RestfulServerTestValidationFailure;
 use SilverStripe\Security\Member;
 use SilverStripe\Security\Security;
 use SilverStripe\ORM\DataObject;
@@ -572,4 +573,17 @@ class RestfulServerTest extends SapphireTest
         unset($_SERVER['PHP_AUTH_USER']);
         unset($_SERVER['PHP_AUTH_PW']);
     }
+
+    public function testValidationErrorWithPOST()
+    {
+        $urlSafeClassname = $this->urlSafeClassname(RestfulServerTestValidationFailure::class);
+        $url = "{$this->baseURI}/api/v1/$urlSafeClassname/";
+        $data = [
+            'Content' => 'Test',
+        ];
+        $response = Director::test($url, $data, null, 'POST');
+        // Assumption: XML is default output
+        $responseArr = Convert::xml2array($response->getBody());
+        $this->assertEquals('SilverStripe\\ORM\\ValidationException', $responseArr['type']);
+    }
 }
diff --git a/tests/unit/Stubs/RestfulServerTestValidationFailure.php b/tests/unit/Stubs/RestfulServerTestValidationFailure.php
new file mode 100644
index 0000000..32e1871
--- /dev/null
+++ b/tests/unit/Stubs/RestfulServerTestValidationFailure.php
@@ -0,0 +1,63 @@
+<?php
+
+namespace SilverStripe\RestfulServer\Tests\Stubs;
+
+use SilverStripe\Dev\TestOnly;
+use SilverStripe\ORM\DataObject;
+
+/**
+ * Class RestfulServerTestValidationFailure
+ * @package SilverStripe\RestfulServer\Tests\Stubs
+ *
+ * @property string Content
+ * @property string Title
+ */
+class RestfulServerTestValidationFailure extends DataObject implements TestOnly
+{
+    private static $api_access = true;
+
+    private static $table_name = 'RestfulServerTestValidationFailure';
+
+    private static $db = array(
+        'Content' => 'Text',
+        'Title' => 'Text',
+    );
+
+    /**
+     * @return \SilverStripe\ORM\ValidationResult
+     */
+    public function validate()
+    {
+        $result = parent::validate();
+
+        if (strlen($this->Content) === 0) {
+            $result->addFieldError('Content', 'Content required');
+        }
+
+        if (strlen($this->Title) === 0) {
+            $result->addFieldError('Title', 'Title required');
+        }
+
+        return $result;
+    }
+
+    public function canView($member = null)
+    {
+        return true;
+    }
+
+    public function canEdit($member = null)
+    {
+        return true;
+    }
+
+    public function canDelete($member = null)
+    {
+        return true;
+    }
+
+    public function canCreate($member = null, $context = array())
+    {
+        return true;
+    }
+}