diff --git a/README.md b/README.md
index c1e636b..21dc5bb 100644
--- a/README.md
+++ b/README.md
@@ -181,7 +181,7 @@ It has support for generating & sending documents with:
 - resource collections
 - to-one and to-many relationships
 - errors (easily turning exceptions into jsonapi output)
-- v1.1 extensions via profiles
+- v1.1 extensions and profiles
 - v1.1 @-members for JSON-LD and others
 
 Also there's tools to help processing of incoming requests:
@@ -189,9 +189,10 @@ Also there's tools to help processing of incoming requests:
 - parse request options (include paths, sparse fieldsets, sort fields, pagination, filtering)
 - parse request documents for creating, updating and deleting resources and relationships
 
-Next to custom extensions, the following [official extensions](https://jsonapi.org/extensions/) are included:
+Next to custom extensions/profiles, the following [official extensions/profiles](https://jsonapi.org/extensions/) are included:
 
-- Cursor Pagination ([example code](/examples/cursor_pagination_profile.php), [specification](https://jsonapi.org/profiles/ethanresnick/cursor-pagination/))
+- Atomic Operations extension ([example code](/examples/atomic_operations_extension.php), [specification](https://jsonapi.org/ext/atomic/))
+- Cursor Pagination profile ([example code](/examples/cursor_pagination_profile.php), [specification](https://jsonapi.org/profiles/ethanresnick/cursor-pagination/))
 
 Plans for the future include:
 
diff --git a/examples/atomic_operations_extension.php b/examples/atomic_operations_extension.php
new file mode 100644
index 0000000..4e97c6d
--- /dev/null
+++ b/examples/atomic_operations_extension.php
@@ -0,0 +1,33 @@
+<?php
+
+use alsvanzelf\jsonapi\objects\ResourceObject;
+use alsvanzelf\jsonapi\extensions\AtomicOperationsDocument;
+
+require 'bootstrap_examples.php';
+
+/**
+ * use the atomic operations extension as extension to the document
+ */
+
+$document = new AtomicOperationsDocument();
+
+$user1  = new ResourceObject('user', 1);
+$user2  = new ResourceObject('user', 2);
+$user42 = new ResourceObject('user', 42);
+
+$user1->add('name',  'Ford');
+$user2->add('name',  'Arthur');
+$user42->add('name', 'Zaphod');
+
+$document->addResults($user1);
+$document->addResults($user2);
+$document->addResults($user42);
+
+/**
+ * get the json
+ */
+
+$options = [
+	'prettyPrint' => true,
+];
+echo '<pre>'.$document->toJson($options);
diff --git a/examples/bootstrap_examples.php b/examples/bootstrap_examples.php
index 40bc6cd..fc4793d 100644
--- a/examples/bootstrap_examples.php
+++ b/examples/bootstrap_examples.php
@@ -2,6 +2,7 @@
 
 use alsvanzelf\jsonapi\Document;
 use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\interfaces\ExtensionInterface;
 use alsvanzelf\jsonapi\interfaces\ProfileInterface;
 use alsvanzelf\jsonapi\interfaces\ResourceInterface;
 
@@ -101,25 +102,59 @@ function getCurrentLocation() {
 	}
 }
 
-class ExampleVersionProfile implements ProfileInterface {
+class ExampleVersionExtension implements ExtensionInterface {
 	/**
 	 * the required method
 	 */
 	
 	public function getOfficialLink() {
-		return 'https://jsonapi.org/format/1.1/#profile-keywords';
+		return 'https://jsonapi.org/format/1.1/#extension-rules';
+	}
+	
+	public function getNamespace() {
+		return 'version';
 	}
 	
 	/**
-	 * optionally helpers for the specific profile
+	 * optionally helpers for the specific extension
 	 */
 	
 	public function setVersion(ResourceInterface $resource, $version) {
 		if ($resource instanceof ResourceDocument) {
-			$resource->addMeta('version', $version, $level=Document::LEVEL_RESOURCE);
+			$resource->getResource()->addExtensionMember($this, 'id', $version);
 		}
 		else {
-			$resource->addMeta('version', $version);
+			$resource->addExtensionMember($this, 'id', $version);
+		}
+	}
+}
+
+class ExampleTimestampsProfile implements ProfileInterface {
+	/**
+	 * the required method
+	 */
+	
+	public function getOfficialLink() {
+		return 'https://jsonapi.org/recommendations/#authoring-profiles';
+	}
+	
+	/**
+	 * optionally helpers for the specific profile
+	 */
+	
+	public function setTimestamps(ResourceInterface $resource, \DateTimeInterface $created=null, \DateTimeInterface $updated=null) {
+		if ($resource instanceof ResourceIdentifierObject) {
+			throw new Exception('cannot add attributes to identifier objects');
+		}
+		
+		$timestamps = [];
+		if ($created !== null) {
+			$timestamps['created'] = $created->format(\DateTime::ISO8601);
 		}
+		if ($updated !== null) {
+			$timestamps['updated'] = $updated->format(\DateTime::ISO8601);
+		}
+		
+		$resource->add('timestamps', $timestamps);
 	}
 }
diff --git a/examples/example_profile.php b/examples/example_profile.php
deleted file mode 100644
index ac12aa1..0000000
--- a/examples/example_profile.php
+++ /dev/null
@@ -1,30 +0,0 @@
-<?php
-
-use alsvanzelf\jsonapi\ResourceDocument;
-
-require 'bootstrap_examples.php';
-
-/**
- * use a profile as extension to the document
- */
-
-$profile = new ExampleVersionProfile();
-
-$document = new ResourceDocument('user', 42);
-$document->applyProfile($profile);
-
-/**
- * you can apply the rules of the profile manually
- * or use methods of the profile if provided
- */
-
-$profile->setVersion($document, '2019');
-
-/**
- * get the json
- */
-
-$options = [
-	'prettyPrint' => true,
-];
-echo '<pre>'.$document->toJson($options);
diff --git a/examples/extension.php b/examples/extension.php
new file mode 100644
index 0000000..74738bf
--- /dev/null
+++ b/examples/extension.php
@@ -0,0 +1,37 @@
+<?php
+
+use alsvanzelf\jsonapi\Document;
+use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\helpers\Converter;
+
+require 'bootstrap_examples.php';
+
+/**
+ * use an extension extend the document with new members
+ */
+
+$extension = new ExampleVersionExtension();
+
+$document = new ResourceDocument('user', 42);
+$document->applyExtension($extension);
+
+$document->add('foo', 'bar');
+
+/**
+ * you can apply the rules of the extension manually
+ * or use methods of the extension if provided
+ */
+
+$extension->setVersion($document, '2019');
+
+/**
+ * get the json
+ */
+
+$contentType = Converter::prepareContentType(Document::CONTENT_TYPE_OFFICIAL, [$extension], []);
+echo '<code>Content-Type: '.$contentType.'</code>'.PHP_EOL;
+
+$options = [
+	'prettyPrint' => true,
+];
+echo '<pre>'.$document->toJson($options);
diff --git a/examples/index.html b/examples/index.html
index 8c1963c..1687b28 100644
--- a/examples/index.html
+++ b/examples/index.html
@@ -49,7 +49,9 @@ <h3>Misc</h3>
 		<li><a href="null_values.php">Null values if explicitly not available</a></li>
 		<li><a href="meta_only.php">Meta-only use-cases</a></li>
 		<li><a href="status_only.php">Status-only</a></li>
-		<li><a href="example_profile.php">Example profile</a></li>
+		<li><a href="extension.php">Example extension</a></li>
+		<li><a href="atomic_operations_extension.php">Atomic operations extension</a></li>
+		<li><a href="profile.php">Example profile</a></li>
 		<li><a href="cursor_pagination_profile.php">Cursor pagination profile</a></li>
 		<li><a href="output.php">Different ways to output</a></li>
 	</ul>
diff --git a/examples/profile.php b/examples/profile.php
new file mode 100644
index 0000000..8b070b6
--- /dev/null
+++ b/examples/profile.php
@@ -0,0 +1,39 @@
+<?php
+
+use alsvanzelf\jsonapi\Document;
+use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\helpers\Converter;
+
+require 'bootstrap_examples.php';
+
+/**
+ * use a profile to define rules for members
+ */
+
+$profile = new ExampleTimestampsProfile();
+
+$document = new ResourceDocument('user', 42);
+$document->applyProfile($profile);
+
+$document->add('foo', 'bar');
+
+/**
+ * you can apply the rules of the profile manually
+ * or use methods of the profile if provided
+ */
+
+$created = new \DateTime('-1 year');
+$updated = new \DateTime('-1 month');
+$profile->setTimestamps($document, $created, $updated);
+
+/**
+ * get the json
+ */
+
+$contentType = Converter::prepareContentType(Document::CONTENT_TYPE_OFFICIAL, [], [$profile]);
+echo '<code>Content-Type: '.$contentType.'</code>'.PHP_EOL;
+
+$options = [
+	'prettyPrint' => true,
+];
+echo '<pre>'.$document->toJson($options);
diff --git a/src/Document.php b/src/Document.php
index 5085e2f..cafa441 100644
--- a/src/Document.php
+++ b/src/Document.php
@@ -2,14 +2,17 @@
 
 namespace alsvanzelf\jsonapi;
 
+use alsvanzelf\jsonapi\exceptions\DuplicateException;
 use alsvanzelf\jsonapi\exceptions\Exception;
 use alsvanzelf\jsonapi\exceptions\InputException;
 use alsvanzelf\jsonapi\helpers\AtMemberManager;
 use alsvanzelf\jsonapi\helpers\Converter;
+use alsvanzelf\jsonapi\helpers\ExtensionMemberManager;
 use alsvanzelf\jsonapi\helpers\HttpStatusCodeManager;
 use alsvanzelf\jsonapi\helpers\LinksManager;
 use alsvanzelf\jsonapi\helpers\Validator;
 use alsvanzelf\jsonapi\interfaces\DocumentInterface;
+use alsvanzelf\jsonapi\interfaces\ExtensionInterface;
 use alsvanzelf\jsonapi\interfaces\ProfileInterface;
 use alsvanzelf\jsonapi\objects\JsonapiObject;
 use alsvanzelf\jsonapi\objects\LinksObject;
@@ -19,7 +22,7 @@
  * @see ResourceDocument, CollectionDocument, ErrorsDocument or MetaDocument
  */
 abstract class Document implements DocumentInterface, \JsonSerializable {
-	use AtMemberManager, HttpStatusCodeManager, LinksManager;
+	use AtMemberManager, ExtensionMemberManager, HttpStatusCodeManager, LinksManager;
 	
 	const JSONAPI_VERSION_1_0 = '1.0';
 	const JSONAPI_VERSION_1_1 = '1.1';
@@ -37,6 +40,8 @@ abstract class Document implements DocumentInterface, \JsonSerializable {
 	protected $meta;
 	/** @var JsonapiObject */
 	protected $jsonapi;
+	/** @var ExtensionInterface[] */
+	protected $extensions = [];
 	/** @var ProfileInterface[] */
 	protected $profiles = [];
 	/** @var array */
@@ -190,6 +195,36 @@ public function unsetJsonapiObject() {
 		$this->jsonapi = null;
 	}
 	
+	/**
+	 * apply a extension which adds the link and sets a correct content-type
+	 * 
+	 * note that the rules from the extension are not automatically enforced
+	 * applying the rules, and applying them correctly, is manual
+	 * however the $extension could have custom methods to help
+	 * 
+	 * @see https://jsonapi.org/extensions/#extensions
+	 * 
+	 * @param ExtensionInterface $extension
+	 * 
+	 * @throws Exception if namespace uses illegal characters
+	 * @throws DuplicateException if namespace conflicts with another applied extension
+	 */
+	public function applyExtension(ExtensionInterface $extension) {
+		$namespace = $extension->getNamespace();
+		if (strlen($namespace) < 1 || preg_match('{[^a-zA-Z0-9]}', $namespace) === 1) {
+			throw new Exception('invalid namespace "'.$namespace.'"');
+		}
+		if (isset($this->extensions[$namespace])) {
+			throw new DuplicateException('an extension with namespace "'.$namespace.'" is already applied');
+		}
+		
+		$this->extensions[$namespace] = $extension;
+		
+		if ($this->jsonapi !== null) {
+			$this->jsonapi->addExtension($extension);
+		}
+	}
+	
 	/**
 	 * apply a profile which adds the link and sets a correct content-type
 	 * 
@@ -197,18 +232,16 @@ public function unsetJsonapiObject() {
 	 * applying the rules, and applying them correctly, is manual
 	 * however the $profile could have custom methods to help
 	 * 
-	 * @see https://jsonapi.org/format/1.1/#profiles
+	 * @see https://jsonapi.org/extensions/#profiles
 	 * 
 	 * @param ProfileInterface $profile
 	 */
 	public function applyProfile(ProfileInterface $profile) {
 		$this->profiles[] = $profile;
 		
-		if ($this->links === null) {
-			$this->setLinksObject(new LinksObject());
+		if ($this->jsonapi !== null) {
+			$this->jsonapi->addProfile($profile);
 		}
-		
-		$this->links->append('profile', $profile->getOfficialLink());
 	}
 	
 	/**
@@ -219,7 +252,14 @@ public function applyProfile(ProfileInterface $profile) {
 	 * @inheritDoc
 	 */
 	public function toArray() {
-		$array = $this->getAtMembers();
+		$array = [];
+		
+		if ($this->hasAtMembers()) {
+			$array = array_merge($array, $this->getAtMembers());
+		}
+		if ($this->hasExtensionMembers()) {
+			$array = array_merge($array, $this->getExtensionMembers());
+		}
 		
 		if ($this->jsonapi !== null && $this->jsonapi->isEmpty() === false) {
 			$array['jsonapi'] = $this->jsonapi->toArray();
@@ -273,7 +313,7 @@ public function sendResponse(array $options=[]) {
 		
 		http_response_code($this->httpStatusCode);
 		
-		$contentType = Converter::mergeProfilesInContentType($options['contentType'], $this->profiles);
+		$contentType = Converter::prepareContentType($options['contentType'], $this->extensions, $this->profiles);
 		header('Content-Type: '.$contentType);
 		
 		echo $json;
diff --git a/src/extensions/AtomicOperationsDocument.php b/src/extensions/AtomicOperationsDocument.php
new file mode 100644
index 0000000..1599fd8
--- /dev/null
+++ b/src/extensions/AtomicOperationsDocument.php
@@ -0,0 +1,57 @@
+<?php
+
+namespace alsvanzelf\jsonapi\extensions;
+
+use alsvanzelf\jsonapi\Document;
+use alsvanzelf\jsonapi\extensions\AtomicOperationsExtension;
+use alsvanzelf\jsonapi\interfaces\DocumentInterface;
+use alsvanzelf\jsonapi\interfaces\ResourceInterface;
+
+/**
+ * document to send results of an atomic operations API
+ */
+class AtomicOperationsDocument extends Document {
+	/** @var AtomicOperationsExtension */
+	private $extension;
+	/** @var ResourceInterface[] */
+	private $results = [];
+	
+	/**
+	 * start the document, auto applies the extension
+	 */
+	public function __construct() {
+		parent::__construct();
+		
+		$this->extension = new AtomicOperationsExtension();
+		$this->applyExtension($this->extension);
+	}
+	
+	/**
+	 * add resources as results of the operations
+	 * 
+	 * @param ResourceInterface[] ...$resources
+	 */
+	public function addResults(ResourceInterface ...$resources) {
+		$this->results = array_merge($this->results, $resources);
+	}
+	
+	/**
+	 * DocumentInterface
+	 */
+	
+	/**
+	 * @inheritDoc
+	 */
+	public function toArray() {
+		$results = [];
+		foreach ($this->results as $result) {
+			$results[] = [
+				'data' => $result->getResource()->toArray(),
+			];
+		}
+		
+		$this->addExtensionMember($this->extension, 'results', $results);
+		
+		return parent::toArray();
+	}
+}
diff --git a/src/extensions/AtomicOperationsExtension.php b/src/extensions/AtomicOperationsExtension.php
new file mode 100644
index 0000000..b94cfe6
--- /dev/null
+++ b/src/extensions/AtomicOperationsExtension.php
@@ -0,0 +1,32 @@
+<?php
+
+namespace alsvanzelf\jsonapi\extensions;
+
+use alsvanzelf\jsonapi\interfaces\ExtensionInterface;
+
+/**
+ * atomic operations provide a means to perform multiple "operations" in a linear and atomic manner
+ * 
+ * @see https://jsonapi.org/ext/atomic/
+ * 
+ * @see AtomicOperationsDocument
+ */
+class AtomicOperationsExtension implements ExtensionInterface {
+	/**
+	 * ExtensionInterface
+	 */
+	
+	/**
+	 * @inheritDoc
+	 */
+	public function getOfficialLink() {
+		return 'https://jsonapi.org/ext/atomic/';
+	}
+	
+	/**
+	 * @inheritDoc
+	 */
+	public function getNamespace() {
+		return 'atomic';
+	}
+}
diff --git a/src/helpers/Converter.php b/src/helpers/Converter.php
index 0d16caf..7d77267 100644
--- a/src/helpers/Converter.php
+++ b/src/helpers/Converter.php
@@ -2,6 +2,7 @@
 
 namespace alsvanzelf\jsonapi\helpers;
 
+use alsvanzelf\jsonapi\interfaces\ExtensionInterface;
 use alsvanzelf\jsonapi\interfaces\ObjectInterface;
 use alsvanzelf\jsonapi\interfaces\ProfileInterface;
 
@@ -34,23 +35,41 @@ public static function camelCaseToWords($camelCase) {
 	}
 	
 	/**
-	 * generates the value for a content type header, with profiles merged in if available
+	 * generates the value for a content type header, with extensions and profiles merged in if available
 	 * 
-	 * @param  string             $contentType
-	 * @param  ProfileInterface[] $profiles
+	 * @param  string               $contentType
+	 * @param  ExtensionInterface[] $extensions
+	 * @param  ProfileInterface[]   $profiles
 	 * @return string
 	 */
-	public static function mergeProfilesInContentType($contentType, array $profiles) {
-		if ($profiles === []) {
-			return $contentType;
+	public static function prepareContentType($contentType, array $extensions, array $profiles) {
+		if ($extensions !== []) {
+			$extensionLinks = [];
+			foreach ($extensions as $extension) {
+				$extensionLinks[] = $extension->getOfficialLink();
+			}
+			$extensionLinks = implode(' ', $extensionLinks);
+			
+			$contentType .= '; ext="'.$extensionLinks.'"';
 		}
 		
-		$profileLinks = [];
-		foreach ($profiles as $profile) {
-			$profileLinks[] = $profile->getOfficialLink();
+		if ($profiles !== []) {
+			$profileLinks = [];
+			foreach ($profiles as $profile) {
+				$profileLinks[] = $profile->getOfficialLink();
+			}
+			$profileLinks = implode(' ', $profileLinks);
+			
+			$contentType .= '; profile="'.$profileLinks.'"';
 		}
-		$profileLinks = implode(' ', $profileLinks);
 		
-		return $contentType.';profile="'.$profileLinks.'", '.$contentType;
+		return $contentType;
+	}
+	
+	/**
+	 * @deprecated {@see prepareContentType()}
+	 */
+	public static function mergeProfilesInContentType($contentType, array $profiles) {
+		return self::prepareContentType($contentType, $extensions=[], $profiles);
 	}
 }
diff --git a/src/helpers/ExtensionMemberManager.php b/src/helpers/ExtensionMemberManager.php
new file mode 100644
index 0000000..4261628
--- /dev/null
+++ b/src/helpers/ExtensionMemberManager.php
@@ -0,0 +1,63 @@
+<?php
+
+namespace alsvanzelf\jsonapi\helpers;
+
+use alsvanzelf\jsonapi\helpers\Converter;
+use alsvanzelf\jsonapi\helpers\Validator;
+use alsvanzelf\jsonapi\interfaces\ExtensionInterface;
+
+trait ExtensionMemberManager {
+	/** @var array */
+	protected $extensionMembers = [];
+	
+	/**
+	 * human api
+	 */
+	
+	/**
+	 * spec api
+	 */
+	
+	/**
+	 * @param ExtensionInterface $extension
+	 * @param string             $key
+	 * @param mixed              $value
+	 */
+	public function addExtensionMember(ExtensionInterface $extension, $key, $value) {
+		$namespace = $extension->getNamespace();
+		
+		if (strpos($key, $namespace.':') === 0) {
+			$key = substr($key, strlen($namespace.':'));
+		}
+		
+		Validator::checkMemberName($key);
+		
+		if (is_object($value)) {
+			$value = Converter::objectToArray($value);
+		}
+		
+		$this->extensionMembers[$namespace.':'.$key] = $value;
+	}
+	
+	/**
+	 * internal api
+	 */
+	
+	/**
+	 * @internal
+	 * 
+	 * @return boolean
+	 */
+	public function hasExtensionMembers() {
+		return ($this->extensionMembers !== []);
+	}
+	
+	/**
+	 * @internal
+	 * 
+	 * @return array
+	 */
+	public function getExtensionMembers() {
+		return $this->extensionMembers;
+	}
+}
diff --git a/src/interfaces/ExtensionInterface.php b/src/interfaces/ExtensionInterface.php
new file mode 100644
index 0000000..3c07497
--- /dev/null
+++ b/src/interfaces/ExtensionInterface.php
@@ -0,0 +1,21 @@
+<?php
+
+namespace alsvanzelf\jsonapi\interfaces;
+
+interface ExtensionInterface {
+	/**
+	 * the unique link identifying and describing the extension
+	 * 
+	 * @internal
+	 * 
+	 * @return string
+	 */
+	public function getOfficialLink();
+	
+	/**
+	 * get the extension's namespace
+	 * 
+	 * @return string
+	 */
+	public function getNamespace();
+}
diff --git a/src/objects/AttributesObject.php b/src/objects/AttributesObject.php
index 5cbc5b1..867c45a 100644
--- a/src/objects/AttributesObject.php
+++ b/src/objects/AttributesObject.php
@@ -4,11 +4,12 @@
 
 use alsvanzelf\jsonapi\helpers\AtMemberManager;
 use alsvanzelf\jsonapi\helpers\Converter;
+use alsvanzelf\jsonapi\helpers\ExtensionMemberManager;
 use alsvanzelf\jsonapi\helpers\Validator;
 use alsvanzelf\jsonapi\interfaces\ObjectInterface;
 
 class AttributesObject implements ObjectInterface {
-	use AtMemberManager;
+	use AtMemberManager, ExtensionMemberManager;
 	
 	/** @var array */
 	protected $attributes = [];
@@ -85,13 +86,32 @@ public function getKeys() {
 	 * @inheritDoc
 	 */
 	public function isEmpty() {
-		return ($this->attributes === [] && $this->hasAtMembers() === false);
+		if ($this->attributes !== []) {
+			return false;
+		}
+		if ($this->hasAtMembers()) {
+			return false;
+		}
+		if ($this->hasExtensionMembers()) {
+			return false;
+		}
+		
+		return true;
 	}
 	
 	/**
 	 * @inheritDoc
 	 */
 	public function toArray() {
-		return array_merge($this->getAtMembers(), $this->attributes);
+		$array = [];
+		
+		if ($this->hasAtMembers()) {
+			$array = array_merge($array, $this->getAtMembers());
+		}
+		if ($this->hasExtensionMembers()) {
+			$array = array_merge($array, $this->getExtensionMembers());
+		}
+		
+		return array_merge($array, $this->attributes);
 	}
 }
diff --git a/src/objects/ErrorObject.php b/src/objects/ErrorObject.php
index 1473b04..9f0eade 100644
--- a/src/objects/ErrorObject.php
+++ b/src/objects/ErrorObject.php
@@ -6,13 +6,14 @@
 use alsvanzelf\jsonapi\exceptions\InputException;
 use alsvanzelf\jsonapi\helpers\AtMemberManager;
 use alsvanzelf\jsonapi\helpers\Converter;
+use alsvanzelf\jsonapi\helpers\ExtensionMemberManager;
 use alsvanzelf\jsonapi\helpers\HttpStatusCodeManager;
 use alsvanzelf\jsonapi\helpers\LinksManager;
 use alsvanzelf\jsonapi\helpers\Validator;
 use alsvanzelf\jsonapi\interfaces\ObjectInterface;
 
 class ErrorObject implements ObjectInterface {
-	use AtMemberManager, HttpStatusCodeManager, LinksManager;
+	use AtMemberManager, ExtensionMemberManager, HttpStatusCodeManager, LinksManager;
 	
 	/** @var string */
 	protected $id;
@@ -288,6 +289,9 @@ public function isEmpty() {
 		if ($this->hasAtMembers()) {
 			return false;
 		}
+		if ($this->hasExtensionMembers()) {
+			return false;
+		}
 		
 		return true;
 	}
@@ -296,8 +300,14 @@ public function isEmpty() {
 	 * @inheritDoc
 	 */
 	public function toArray() {
-		$array = $this->getAtMembers();
+		$array = [];
 		
+		if ($this->hasAtMembers()) {
+			$array = array_merge($array, $this->getAtMembers());
+		}
+		if ($this->hasExtensionMembers()) {
+			$array = array_merge($array, $this->getExtensionMembers());
+		}
 		if ($this->id !== null) {
 			$array['id'] = $this->id;
 		}
diff --git a/src/objects/JsonapiObject.php b/src/objects/JsonapiObject.php
index ad04da2..980b0b2 100644
--- a/src/objects/JsonapiObject.php
+++ b/src/objects/JsonapiObject.php
@@ -4,14 +4,21 @@
 
 use alsvanzelf\jsonapi\Document;
 use alsvanzelf\jsonapi\helpers\AtMemberManager;
+use alsvanzelf\jsonapi\helpers\ExtensionMemberManager;
+use alsvanzelf\jsonapi\interfaces\ExtensionInterface;
 use alsvanzelf\jsonapi\interfaces\ObjectInterface;
+use alsvanzelf\jsonapi\interfaces\ProfileInterface;
 use alsvanzelf\jsonapi\objects\MetaObject;
 
 class JsonapiObject implements ObjectInterface {
-	use AtMemberManager;
+	use AtMemberManager, ExtensionMemberManager;
 	
 	/** @var string */
 	protected $version;
+	/** @var ExtensionInterface[] */
+	protected $extensions = [];
+	/** @var ProfileInterface */
+	protected $profiles = [];
 	/** @var MetaObject */
 	protected $meta;
 	
@@ -51,6 +58,20 @@ public function setVersion($version) {
 		$this->version = $version;
 	}
 	
+	/**
+	 * @param ExtensionInterface $extension
+	 */
+	public function addExtension(ExtensionInterface $extension) {
+		$this->extensions[] = $extension;
+	}
+	
+	/**
+	 * @param ProfileInterface $profile
+	 */
+	public function addProfile(ProfileInterface $profile) {
+		$this->profiles[] = $profile;
+	}
+	
 	/**
 	 * @param MetaObject $metaObject
 	 */
@@ -69,12 +90,21 @@ public function isEmpty() {
 		if ($this->version !== null) {
 			return false;
 		}
+		if ($this->extensions !== []) {
+			return false;
+		}
+		if ($this->profiles !== []) {
+			return false;
+		}
 		if ($this->meta !== null && $this->meta->isEmpty() === false) {
 			return false;
 		}
 		if ($this->hasAtMembers()) {
 			return false;
 		}
+		if ($this->hasExtensionMembers()) {
+			return false;
+		}
 		
 		return true;
 	}
@@ -83,11 +113,29 @@ public function isEmpty() {
 	 * @inheritDoc
 	 */
 	public function toArray() {
-		$array = $this->getAtMembers();
+		$array = [];
 		
+		if ($this->hasAtMembers()) {
+			$array = array_merge($array, $this->getAtMembers());
+		}
+		if ($this->hasExtensionMembers()) {
+			$array = array_merge($array, $this->getExtensionMembers());
+		}
 		if ($this->version !== null) {
 			$array['version'] = $this->version;
 		}
+		if ($this->extensions !== []) {
+			$array['ext'] = [];
+			foreach ($this->extensions as $extension) {
+				$array['ext'][] = $extension->getOfficialLink();
+			}
+		}
+		if ($this->profiles !== []) {
+			$array['profile'] = [];
+			foreach ($this->profiles as $profile) {
+				$array['profile'][] = $profile->getOfficialLink();
+			}
+		}
 		if ($this->meta !== null && $this->meta->isEmpty() === false) {
 			$array['meta'] = $this->meta->toArray();
 		}
diff --git a/src/objects/LinkObject.php b/src/objects/LinkObject.php
index 837875e..244d103 100644
--- a/src/objects/LinkObject.php
+++ b/src/objects/LinkObject.php
@@ -3,11 +3,12 @@
 namespace alsvanzelf\jsonapi\objects;
 
 use alsvanzelf\jsonapi\helpers\AtMemberManager;
+use alsvanzelf\jsonapi\helpers\ExtensionMemberManager;
 use alsvanzelf\jsonapi\interfaces\ObjectInterface;
 use alsvanzelf\jsonapi\objects\MetaObject;
 
 class LinkObject implements ObjectInterface {
-	use AtMemberManager;
+	use AtMemberManager, ExtensionMemberManager;
 	
 	/** @var string */
 	protected $href;
@@ -161,6 +162,9 @@ public function isEmpty() {
 		if ($this->hasAtMembers()) {
 			return false;
 		}
+		if ($this->hasExtensionMembers()) {
+			return false;
+		}
 		
 		return true;
 	}
@@ -169,7 +173,14 @@ public function isEmpty() {
 	 * @inheritDoc
 	 */
 	public function toArray() {
-		$array = $this->getAtMembers();
+		$array = [];
+		
+		if ($this->hasAtMembers()) {
+			$array = array_merge($array, $this->getAtMembers());
+		}
+		if ($this->hasExtensionMembers()) {
+			$array = array_merge($array, $this->getExtensionMembers());
+		}
 		
 		$array['href'] = $this->href;
 		
diff --git a/src/objects/LinksArray.php b/src/objects/LinksArray.php
index d390e01..d4ae042 100644
--- a/src/objects/LinksArray.php
+++ b/src/objects/LinksArray.php
@@ -9,7 +9,6 @@
 /**
  * an array of links (strings and LinkObjects), used for:
  * - type links in an ErrorObject
- * - profile links at root level
  */
 class LinksArray implements ObjectInterface {
 	/** @var array with string|LinkObject */
diff --git a/src/objects/LinksObject.php b/src/objects/LinksObject.php
index 9e6347b..67e9640 100644
--- a/src/objects/LinksObject.php
+++ b/src/objects/LinksObject.php
@@ -5,13 +5,14 @@
 use alsvanzelf\jsonapi\exceptions\DuplicateException;
 use alsvanzelf\jsonapi\helpers\AtMemberManager;
 use alsvanzelf\jsonapi\helpers\Converter;
+use alsvanzelf\jsonapi\helpers\ExtensionMemberManager;
 use alsvanzelf\jsonapi\helpers\Validator;
 use alsvanzelf\jsonapi\interfaces\ObjectInterface;
 use alsvanzelf\jsonapi\objects\LinkObject;
 use alsvanzelf\jsonapi\objects\LinksArray;
 
 class LinksObject implements ObjectInterface {
-	use AtMemberManager;
+	use AtMemberManager, ExtensionMemberManager;
 	
 	/** @var array with string|LinkObject */
 	protected $links = [];
@@ -161,14 +162,31 @@ public function appendLinkObject($key, LinkObject $linkObject) {
 	 * @inheritDoc
 	 */
 	public function isEmpty() {
-		return ($this->links === [] && $this->hasAtMembers() === false);
+		if ($this->links !== []) {
+			return false;
+		}
+		if ($this->hasAtMembers()) {
+			return false;
+		}
+		if ($this->hasExtensionMembers()) {
+			return false;
+		}
+		
+		return true;
 	}
 	
 	/**
 	 * @inheritDoc
 	 */
 	public function toArray() {
-		$array = $this->getAtMembers();
+		$array = [];
+		
+		if ($this->hasAtMembers()) {
+			$array = array_merge($array, $this->getAtMembers());
+		}
+		if ($this->hasExtensionMembers()) {
+			$array = array_merge($array, $this->getExtensionMembers());
+		}
 		
 		foreach ($this->links as $key => $link) {
 			if ($link instanceof LinkObject && $link->isEmpty() === false) {
diff --git a/src/objects/MetaObject.php b/src/objects/MetaObject.php
index f31eb28..03e9e1e 100644
--- a/src/objects/MetaObject.php
+++ b/src/objects/MetaObject.php
@@ -4,11 +4,12 @@
 
 use alsvanzelf\jsonapi\helpers\AtMemberManager;
 use alsvanzelf\jsonapi\helpers\Converter;
+use alsvanzelf\jsonapi\helpers\ExtensionMemberManager;
 use alsvanzelf\jsonapi\helpers\Validator;
 use alsvanzelf\jsonapi\interfaces\ObjectInterface;
 
 class MetaObject implements ObjectInterface {
-	use AtMemberManager;
+	use AtMemberManager, ExtensionMemberManager;
 	
 	/** @var array */
 	protected $meta = [];
@@ -67,13 +68,32 @@ public function add($key, $value) {
 	 * @inheritDoc
 	 */
 	public function isEmpty() {
-		return ($this->meta === [] && $this->hasAtMembers() === false);
+		if ($this->meta !== []) {
+			return false;
+		}
+		if ($this->hasAtMembers()) {
+			return false;
+		}
+		if ($this->hasExtensionMembers()) {
+			return false;
+		}
+		
+		return true;
 	}
 	
 	/**
 	 * @inheritDoc
 	 */
 	public function toArray() {
-		return array_merge($this->getAtMembers(), $this->meta);
+		$array = [];
+		
+		if ($this->hasAtMembers()) {
+			$array = array_merge($array, $this->getAtMembers());
+		}
+		if ($this->hasExtensionMembers()) {
+			$array = array_merge($array, $this->getExtensionMembers());
+		}
+		
+		return array_merge($array, $this->meta);
 	}
 }
diff --git a/src/objects/RelationshipObject.php b/src/objects/RelationshipObject.php
index ea62561..7644cd5 100644
--- a/src/objects/RelationshipObject.php
+++ b/src/objects/RelationshipObject.php
@@ -5,6 +5,7 @@
 use alsvanzelf\jsonapi\CollectionDocument;
 use alsvanzelf\jsonapi\exceptions\InputException;
 use alsvanzelf\jsonapi\helpers\AtMemberManager;
+use alsvanzelf\jsonapi\helpers\ExtensionMemberManager;
 use alsvanzelf\jsonapi\helpers\LinksManager;
 use alsvanzelf\jsonapi\interfaces\ObjectInterface;
 use alsvanzelf\jsonapi\interfaces\PaginableInterface;
@@ -15,7 +16,7 @@
 use alsvanzelf\jsonapi\objects\ResourceObject;
 
 class RelationshipObject implements ObjectInterface, PaginableInterface, RecursiveResourceContainerInterface {
-	use AtMemberManager, LinksManager;
+	use AtMemberManager, ExtensionMemberManager, LinksManager;
 	
 	const TO_ONE  = 'one';
 	const TO_MANY = 'many';
@@ -273,6 +274,9 @@ public function isEmpty() {
 		if ($this->hasAtMembers()) {
 			return false;
 		}
+		if ($this->hasExtensionMembers()) {
+			return false;
+		}
 		
 		return true;
 	}
@@ -281,7 +285,14 @@ public function isEmpty() {
 	 * @inheritDoc
 	 */
 	public function toArray() {
-		$array = $this->getAtMembers();
+		$array = [];
+		
+		if ($this->hasAtMembers()) {
+			$array = array_merge($array, $this->getAtMembers());
+		}
+		if ($this->hasExtensionMembers()) {
+			$array = array_merge($array, $this->getExtensionMembers());
+		}
 		
 		if ($this->links !== null && $this->links->isEmpty() === false) {
 			$array['links'] = $this->links->toArray();
diff --git a/src/objects/RelationshipsObject.php b/src/objects/RelationshipsObject.php
index 6764aa3..c0bf3e0 100644
--- a/src/objects/RelationshipsObject.php
+++ b/src/objects/RelationshipsObject.php
@@ -4,6 +4,7 @@
 
 use alsvanzelf\jsonapi\exceptions\DuplicateException;
 use alsvanzelf\jsonapi\helpers\AtMemberManager;
+use alsvanzelf\jsonapi\helpers\ExtensionMemberManager;
 use alsvanzelf\jsonapi\helpers\Validator;
 use alsvanzelf\jsonapi\interfaces\ObjectInterface;
 use alsvanzelf\jsonapi\interfaces\RecursiveResourceContainerInterface;
@@ -12,7 +13,7 @@
 use alsvanzelf\jsonapi\objects\ResourceObject;
 
 class RelationshipsObject implements ObjectInterface, RecursiveResourceContainerInterface {
-	use AtMemberManager;
+	use AtMemberManager, ExtensionMemberManager;
 	
 	/** @var RelationshipObject[] */
 	protected $relationships = [];
@@ -77,14 +78,31 @@ public function getKeys() {
 	 * @inheritDoc
 	 */
 	public function isEmpty() {
-		return ($this->relationships === [] && $this->hasAtMembers() === false);
+		if ($this->relationships !== []) {
+			return false;
+		}
+		if ($this->hasAtMembers()) {
+			return false;
+		}
+		if ($this->hasExtensionMembers()) {
+			return false;
+		}
+		
+		return true;
 	}
 	
 	/**
 	 * @inheritDoc
 	 */
 	public function toArray() {
-		$array = $this->getAtMembers();
+		$array = [];
+		
+		if ($this->hasAtMembers()) {
+			$array = array_merge($array, $this->getAtMembers());
+		}
+		if ($this->hasExtensionMembers()) {
+			$array = array_merge($array, $this->getExtensionMembers());
+		}
 		
 		foreach ($this->relationships as $key => $relationshipObject) {
 			$array[$key] = $relationshipObject->toArray();
diff --git a/src/objects/ResourceIdentifierObject.php b/src/objects/ResourceIdentifierObject.php
index a06c33f..05aaa6b 100644
--- a/src/objects/ResourceIdentifierObject.php
+++ b/src/objects/ResourceIdentifierObject.php
@@ -5,13 +5,14 @@
 use alsvanzelf\jsonapi\exceptions\Exception;
 use alsvanzelf\jsonapi\exceptions\DuplicateException;
 use alsvanzelf\jsonapi\helpers\AtMemberManager;
+use alsvanzelf\jsonapi\helpers\ExtensionMemberManager;
 use alsvanzelf\jsonapi\helpers\Validator;
 use alsvanzelf\jsonapi\interfaces\ObjectInterface;
 use alsvanzelf\jsonapi\interfaces\ResourceInterface;
 use alsvanzelf\jsonapi\objects\MetaObject;
 
 class ResourceIdentifierObject implements ObjectInterface, ResourceInterface {
-	use AtMemberManager;
+	use AtMemberManager, ExtensionMemberManager;
 	
 	/** @var string */
 	protected $type;
@@ -191,6 +192,9 @@ public function isEmpty() {
 		if ($this->hasAtMembers()) {
 			return false;
 		}
+		if ($this->hasExtensionMembers()) {
+			return false;
+		}
 		
 		return true;
 	}
@@ -199,7 +203,7 @@ public function isEmpty() {
 	 * @inheritDoc
 	 */
 	public function toArray() {
-		$array = $this->getAtMembers();
+		$array = [];
 		
 		$array['type'] = $this->type;
 		
@@ -210,6 +214,13 @@ public function toArray() {
 			$array['lid'] = $this->lid;
 		}
 		
+		if ($this->hasAtMembers()) {
+			$array = array_merge($array, $this->getAtMembers());
+		}
+		if ($this->hasExtensionMembers()) {
+			$array = array_merge($array, $this->getExtensionMembers());
+		}
+		
 		if ($this->meta !== null && $this->meta->isEmpty() === false) {
 			$array['meta'] = $this->meta->toArray();
 		}
diff --git a/tests/ConverterTest.php b/tests/ConverterTest.php
index 85839c9..f5c7d51 100644
--- a/tests/ConverterTest.php
+++ b/tests/ConverterTest.php
@@ -4,6 +4,7 @@
 
 use alsvanzelf\jsonapi\helpers\Converter;
 use alsvanzelf\jsonapi\objects\AttributesObject;
+use alsvanzelf\jsonapiTests\extensions\TestExtension;
 use alsvanzelf\jsonapiTests\profiles\TestProfile;
 use PHPUnit\Framework\TestCase;
 
@@ -63,25 +64,63 @@ public function dataProviderCamelCaseToWords_HappyPath() {
 		];
 	}
 	
-	public function testMergeProfilesInContentType_HappyPath() {
-		$this->assertSame('foo', Converter::mergeProfilesInContentType('foo', []));
+	/**
+	 * @group Extensions
+	 * @group Profiles
+	 */
+	public function testPrepareContentType_HappyPath() {
+		$this->assertSame('foo', Converter::prepareContentType('foo', [], []));
 	}
 	
-	public function testMergeProfilesInContentType_WithProfileStringLink() {
+	/**
+	 * @group Extensions
+	 */
+	public function testPrepareContentType_WithExtensionStringLink() {
+		$extension = new TestExtension();
+		$extension->setOfficialLink('bar');
+		
+		$this->assertSame('foo; ext="bar"', Converter::prepareContentType('foo', [$extension], []));
+	}
+	
+	/**
+	 * @group Profiles
+	 */
+	public function testPrepareContentType_WithProfileStringLink() {
 		$profile = new TestProfile();
 		$profile->setOfficialLink('bar');
 		
-		$this->assertSame('foo;profile="bar", foo', Converter::mergeProfilesInContentType('foo', [$profile]));
+		$this->assertSame('foo; profile="bar"', Converter::prepareContentType('foo', [], [$profile]));
 	}
 	
-	public function testMergeProfilesInContentType_WithMultipleProfiles() {
+	/**
+	 * @group Extensions
+	 * @group Profiles
+	 */
+	public function testPrepareContentType_WithMultipleExtensionsAndProfiles() {
+		$extension1 = new TestExtension();
+		$extension1->setOfficialLink('bar');
+		
+		$extension2 = new TestExtension();
+		$extension2->setOfficialLink('baz');
+		
 		$profile1 = new TestProfile();
 		$profile1->setOfficialLink('bar');
 		
 		$profile2 = new TestProfile();
 		$profile2->setOfficialLink('baz');
 		
-		$this->assertSame('foo;profile="bar baz", foo', Converter::mergeProfilesInContentType('foo', [$profile1, $profile2]));
+		$this->assertSame('foo; ext="bar baz"; profile="bar baz"', Converter::prepareContentType('foo', [$extension1, $extension2], [$profile1, $profile2]));
+	}
+	
+	/**
+	 * test method while it is part of the interface
+	 * @group Profiles
+	 */
+	public function testMergeProfilesInContentType_HappyPath() {
+		$profile = new TestProfile();
+		$profile->setOfficialLink('bar');
+		
+		$this->assertSame('foo; profile="bar"', Converter::mergeProfilesInContentType('foo', [$profile]));
 	}
 }
 
diff --git a/tests/DocumentTest.php b/tests/DocumentTest.php
index d363b8e..30043cb 100644
--- a/tests/DocumentTest.php
+++ b/tests/DocumentTest.php
@@ -2,10 +2,12 @@
 
 namespace alsvanzelf\jsonapiTests;
 
+use alsvanzelf\jsonapi\exceptions\DuplicateException;
 use alsvanzelf\jsonapi\exceptions\Exception;
 use alsvanzelf\jsonapi\exceptions\InputException;
 use alsvanzelf\jsonapi\objects\LinkObject;
 use alsvanzelf\jsonapiTests\TestableNonAbstractDocument as Document;
+use alsvanzelf\jsonapiTests\extensions\TestExtension;
 use alsvanzelf\jsonapiTests\profiles\TestProfile;
 use PHPUnit\Framework\TestCase;
 
@@ -229,6 +231,71 @@ public function testAddLinkObject_HappyPath() {
 		$this->assertSame('https://jsonapi.org', $array['links']['foo']['href']);
 	}
 	
+	/**
+	 * @group Extensions
+	 */
+	public function testApplyExtension_HappyPath() {
+		$extension = new TestExtension();
+		$extension->setNamespace('test');
+		$extension->setOfficialLink('https://jsonapi.org');
+		
+		$document = new Document();
+		$document->applyExtension($extension);
+		$document->addExtensionMember($extension, 'foo', 'bar');
+		
+		$array = $document->toArray();
+		
+		$this->assertArrayHasKey('jsonapi', $array);
+		$this->assertCount(2, $array['jsonapi']);
+		$this->assertSame('1.1', $array['jsonapi']['version']);
+		$this->assertArrayHasKey('ext', $array['jsonapi']);
+		$this->assertCount(1, $array['jsonapi']['ext']);
+		$this->assertArrayHasKey(0, $array['jsonapi']['ext']);
+		$this->assertSame('https://jsonapi.org', $array['jsonapi']['ext'][0]);
+		$this->assertArrayHasKey('test:foo', $array);
+		$this->assertSame('bar', $array['test:foo']);
+	}
+	
+	/**
+	 * @group Extensions
+	 */
+	public function testApplyExtension_InvalidNamespace() {
+		$document  = new Document();
+		$extension = new TestExtension();
+		$extension->setNamespace('foo-bar');
+		
+		$this->expectException(Exception::class);
+		$this->expectExceptionMessage('invalid namespace "foo-bar"');
+		
+		$document->applyExtension($extension);
+	}
+	
+	/**
+	 * @group Extensions
+	 */
+	public function testApplyExtension_ConflictingNamespace() {
+		$document  = new Document();
+		
+		$extension1 = new TestExtension();
+		$extension1->setNamespace('foo');
+		$document->applyExtension($extension1);
+		
+		$extension2 = new TestExtension();
+		$extension2->setNamespace('bar');
+		$document->applyExtension($extension2);
+		
+		$extension3 = new TestExtension();
+		$extension3->setNamespace('foo');
+		
+		$this->expectException(DuplicateException::class);
+		$this->expectExceptionMessage('an extension with namespace "foo" is already applied');
+		
+		$document->applyExtension($extension3);
+	}
+	
+	/**
+	 * @group Profiles
+	 */
 	public function testApplyProfile_HappyPath() {
 		$profile = new TestProfile();
 		$profile->setOfficialLink('https://jsonapi.org');
@@ -238,12 +305,13 @@ public function testApplyProfile_HappyPath() {
 		
 		$array = $document->toArray();
 		
-		$this->assertArrayHasKey('links', $array);
-		$this->assertCount(1, $array['links']);
-		$this->assertArrayHasKey('profile', $array['links']);
-		$this->assertCount(1, $array['links']['profile']);
-		$this->assertArrayHasKey(0, $array['links']['profile']);
-		$this->assertSame('https://jsonapi.org', $array['links']['profile'][0]);
+		$this->assertArrayHasKey('jsonapi', $array);
+		$this->assertCount(2, $array['jsonapi']);
+		$this->assertSame('1.1', $array['jsonapi']['version']);
+		$this->assertArrayHasKey('profile', $array['jsonapi']);
+		$this->assertCount(1, $array['jsonapi']['profile']);
+		$this->assertArrayHasKey(0, $array['jsonapi']['profile']);
+		$this->assertSame('https://jsonapi.org', $array['jsonapi']['profile'][0]);
 	}
 	
 	public function testToJson_HappyPath() {
diff --git a/tests/SeparateProcessTest.php b/tests/SeparateProcessTest.php
index d81453f..ec00ccd 100644
--- a/tests/SeparateProcessTest.php
+++ b/tests/SeparateProcessTest.php
@@ -3,6 +3,7 @@
 namespace alsvanzelf\jsonapiTests;
 
 use alsvanzelf\jsonapiTests\TestableNonAbstractDocument as Document;
+use alsvanzelf\jsonapiTests\extensions\TestExtension;
 use alsvanzelf\jsonapiTests\profiles\TestProfile;
 use PHPUnit\Framework\TestCase;
 
@@ -74,6 +75,39 @@ public function testSendResponse_ContentTypeHeader() {
 	
 	/**
 	 * @runInSeparateProcess
+	 * @group Extensions
+	 */
+	public function testSendResponse_ContentTypeHeaderWithExtensions() {
+		if (extension_loaded('xdebug') === false) {
+			$this->markTestSkipped('can not run without xdebug');
+		}
+		
+		$extension = new TestExtension();
+		$extension->setNamespace('one');
+		$extension->setOfficialLink('https://jsonapi.org');
+		
+		$document = new Document();
+		$document->applyExtension($extension);
+		
+		ob_start();
+		$document->sendResponse();
+		ob_end_clean();
+		$this->assertSame(['Content-Type: '.Document::CONTENT_TYPE_OFFICIAL.'; ext="https://jsonapi.org"'], xdebug_get_headers());
+		
+		$extension = new TestExtension();
+		$extension->setNamespace('two');
+		$extension->setOfficialLink('https://jsonapi.org/2');
+		$document->applyExtension($extension);
+		
+		ob_start();
+		$document->sendResponse();
+		ob_end_clean();
+		$this->assertSame(['Content-Type: '.Document::CONTENT_TYPE_OFFICIAL.'; ext="https://jsonapi.org https://jsonapi.org/2"'], xdebug_get_headers());
+	}
+	
+	/**
+	 * @runInSeparateProcess
+	 * @group Profiles
 	 */
 	public function testSendResponse_ContentTypeHeaderWithProfiles() {
 		if (extension_loaded('xdebug') === false) {
@@ -89,7 +123,7 @@ public function testSendResponse_ContentTypeHeaderWithProfiles() {
 		ob_start();
 		$document->sendResponse();
 		ob_end_clean();
-		$this->assertSame(['Content-Type: '.Document::CONTENT_TYPE_OFFICIAL.';profile="https://jsonapi.org", '.Document::CONTENT_TYPE_OFFICIAL], xdebug_get_headers());
+		$this->assertSame(['Content-Type: '.Document::CONTENT_TYPE_OFFICIAL.'; profile="https://jsonapi.org"'], xdebug_get_headers());
 		
 		$profile = new TestProfile();
 		$profile->setOfficialLink('https://jsonapi.org/2');
@@ -98,7 +132,7 @@ public function testSendResponse_ContentTypeHeaderWithProfiles() {
 		ob_start();
 		$document->sendResponse();
 		ob_end_clean();
-		$this->assertSame(['Content-Type: '.Document::CONTENT_TYPE_OFFICIAL.';profile="https://jsonapi.org https://jsonapi.org/2", '.Document::CONTENT_TYPE_OFFICIAL], xdebug_get_headers());
+		$this->assertSame(['Content-Type: '.Document::CONTENT_TYPE_OFFICIAL.'; profile="https://jsonapi.org https://jsonapi.org/2"'], xdebug_get_headers());
 	}
 	
 	/**
diff --git a/tests/example_output/ExampleEverywhereExtension.php b/tests/example_output/ExampleEverywhereExtension.php
new file mode 100644
index 0000000..885e26b
--- /dev/null
+++ b/tests/example_output/ExampleEverywhereExtension.php
@@ -0,0 +1,15 @@
+<?php
+
+namespace alsvanzelf\jsonapiTests\example_output;
+
+use alsvanzelf\jsonapi\interfaces\ExtensionInterface;
+
+class ExampleEverywhereExtension implements ExtensionInterface {
+	public function getOfficialLink() {
+		return 'https://example.org/everywhere-extension';
+	}
+	
+	public function getNamespace() {
+		return 'everywhere';
+	}
+}
diff --git a/tests/example_output/ExampleTimestampsProfile.php b/tests/example_output/ExampleTimestampsProfile.php
new file mode 100644
index 0000000..56b07f5
--- /dev/null
+++ b/tests/example_output/ExampleTimestampsProfile.php
@@ -0,0 +1,29 @@
+<?php
+
+namespace alsvanzelf\jsonapiTests\example_output;
+
+use alsvanzelf\jsonapi\interfaces\ProfileInterface;
+use alsvanzelf\jsonapi\interfaces\ResourceInterface;
+use alsvanzelf\jsonapi\objects\ResourceIdentifierObject;
+
+class ExampleTimestampsProfile implements ProfileInterface {
+	public function getOfficialLink() {
+		return 'https://jsonapi.org/recommendations/#authoring-profiles';
+	}
+	
+	public function setTimestamps(ResourceInterface $resource, \DateTimeInterface $created=null, \DateTimeInterface $updated=null) {
+		if ($resource instanceof ResourceIdentifierObject) {
+			throw new Exception('cannot add attributes to identifier objects');
+		}
+		
+		$timestamps = [];
+		if ($created !== null) {
+			$timestamps['created'] = $created->format(\DateTime::ISO8601);
+		}
+		if ($updated !== null) {
+			$timestamps['updated'] = $updated->format(\DateTime::ISO8601);
+		}
+		
+		$resource->add('timestamps', $timestamps);
+	}
+}
diff --git a/tests/example_output/ExampleVersionExtension.php b/tests/example_output/ExampleVersionExtension.php
new file mode 100644
index 0000000..bde454a
--- /dev/null
+++ b/tests/example_output/ExampleVersionExtension.php
@@ -0,0 +1,26 @@
+<?php
+
+namespace alsvanzelf\jsonapiTests\example_output;
+
+use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\interfaces\ExtensionInterface;
+use alsvanzelf\jsonapi\interfaces\ResourceInterface;
+
+class ExampleVersionExtension implements ExtensionInterface {
+	public function getOfficialLink() {
+		return 'https://jsonapi.org/format/1.1/#extension-rules';
+	}
+	
+	public function getNamespace() {
+		return 'version';
+	}
+	
+	public function setVersion(ResourceInterface $resource, $version) {
+		if ($resource instanceof ResourceDocument) {
+			$resource->getResource()->addExtensionMember($this, 'id', $version);
+		}
+		else {
+			$resource->addExtensionMember($this, 'id', $version);
+		}
+	}
+}
diff --git a/tests/example_output/ExampleVersionProfile.php b/tests/example_output/ExampleVersionProfile.php
deleted file mode 100644
index 266be3f..0000000
--- a/tests/example_output/ExampleVersionProfile.php
+++ /dev/null
@@ -1,27 +0,0 @@
-<?php
-
-namespace alsvanzelf\jsonapiTests\example_output;
-
-use alsvanzelf\jsonapi\Document;
-use alsvanzelf\jsonapi\ResourceDocument;
-use alsvanzelf\jsonapi\interfaces\ProfileInterface;
-use alsvanzelf\jsonapi\interfaces\ResourceInterface;
-
-class ExampleVersionProfile implements ProfileInterface {
-	public function getOfficialLink() {
-		return 'https://jsonapi.org/format/1.1/#profile-keywords';
-	}
-	
-	/**
-	 * optionally helpers for the specific profile
-	 */
-	
-	public function setVersion(ResourceInterface $resource, $version) {
-		if ($resource instanceof ResourceDocument) {
-			$resource->addMeta('version', $version, $level=Document::LEVEL_RESOURCE);
-		}
-		else {
-			$resource->addMeta('version', $version);
-		}
-	}
-}
diff --git a/tests/example_output/at_members_everywhere/at_members_everywhere.json b/tests/example_output/at_members_everywhere/at_members_everywhere.json
index feb9aea..76638c7 100644
--- a/tests/example_output/at_members_everywhere/at_members_everywhere.json
+++ b/tests/example_output/at_members_everywhere/at_members_everywhere.json
@@ -21,9 +21,9 @@
 		"@context": "/meta/@context"
 	},
 	"data": {
-		"@context": "/data/@context",
 		"type": "user",
 		"id": "42",
+		"@context": "/data/@context",
 		"meta": {
 			"@context": "/data/meta/@context"
 		},
@@ -48,9 +48,9 @@
 			"bar": {
 				"@context": "/data/relationships/bar/@context",
 				"data": {
-					"@context": "/data/relationships/bar/data/@context",
 					"type": "user",
 					"id": "2",
+					"@context": "/data/relationships/bar/data/@context",
 					"meta": {
 						"@context": "/data/relationships/bar/data/meta/@context"
 					}
@@ -70,17 +70,17 @@
 	},
 	"included": [
 		{
-			"@context": "/included/0/@context",
 			"type": "user",
 			"id": "1",
+			"@context": "/included/0/@context",
 			"attributes": {
 				"@context": "/included/0/attributes/@context"
 			}
 		},
 		{
-			"@context": "/included/1/@context",
 			"type": "user",
 			"id": "3",
+			"@context": "/included/1/@context",
 			"relationships": {
 				"@context": "/included/1/relationships/@context"
 			}
diff --git a/tests/example_output/cursor_pagination_profile/cursor_pagination_profile.json b/tests/example_output/cursor_pagination_profile/cursor_pagination_profile.json
index cb84fda..02b81ad 100644
--- a/tests/example_output/cursor_pagination_profile/cursor_pagination_profile.json
+++ b/tests/example_output/cursor_pagination_profile/cursor_pagination_profile.json
@@ -1,11 +1,11 @@
 {
 	"jsonapi": {
-		"version": "1.1"
-	},
-	"links": {
+		"version": "1.1",
 		"profile": [
 			"https://jsonapi.org/profiles/ethanresnick/cursor-pagination/"
-		],
+		]
+	},
+	"links": {
 		"prev": null,
 		"next": {
 			"href": "/users?sort=42&page[size]=10&page[after]=zaphod"
diff --git a/tests/example_output/example_profile/example_profile.json b/tests/example_output/example_profile/example_profile.json
deleted file mode 100644
index 53daa9f..0000000
--- a/tests/example_output/example_profile/example_profile.json
+++ /dev/null
@@ -1,17 +0,0 @@
-{
-	"jsonapi": {
-		"version": "1.1"
-	},
-	"links": {
-		"profile": [
-			"https://jsonapi.org/format/1.1/#profile-keywords"
-		]
-	},
-	"data": {
-		"type": "user",
-		"id": "42",
-		"meta": {
-			"version": "2019"
-		}
-	}
-}
diff --git a/tests/example_output/example_profile/example_profile.php b/tests/example_output/example_profile/example_profile.php
deleted file mode 100644
index 65b3bb4..0000000
--- a/tests/example_output/example_profile/example_profile.php
+++ /dev/null
@@ -1,19 +0,0 @@
-<?php
-
-namespace alsvanzelf\jsonapiTests\example_output\example_profile;
-
-use alsvanzelf\jsonapi\ResourceDocument;
-use alsvanzelf\jsonapiTests\example_output\ExampleVersionProfile;
-
-class example_profile {
-	public static function createJsonapiDocument() {
-		$profile = new ExampleVersionProfile();
-		
-		$document = new ResourceDocument('user', 42);
-		$document->applyProfile($profile);
-		
-		$profile->setVersion($document, '2019');
-		
-		return $document;
-	}
-}
diff --git a/tests/example_output/extension/extension.json b/tests/example_output/extension/extension.json
new file mode 100644
index 0000000..4b857c0
--- /dev/null
+++ b/tests/example_output/extension/extension.json
@@ -0,0 +1,13 @@
+{
+	"jsonapi": {
+		"version": "1.1",
+		"ext": [
+			"https://jsonapi.org/format/1.1/#extension-rules"
+		]
+	},
+	"data": {
+		"type": "user",
+		"id": "42",
+		"version:id": "2019"
+	}
+}
diff --git a/tests/example_output/extension/extension.php b/tests/example_output/extension/extension.php
new file mode 100644
index 0000000..4535c63
--- /dev/null
+++ b/tests/example_output/extension/extension.php
@@ -0,0 +1,19 @@
+<?php
+
+namespace alsvanzelf\jsonapiTests\example_output\extension;
+
+use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapiTests\example_output\ExampleVersionExtension;
+
+class extension {
+	public static function createJsonapiDocument() {
+		$extension = new ExampleVersionExtension();
+		
+		$document = new ResourceDocument('user', 42);
+		$document->applyExtension($extension);
+		
+		$extension->setVersion($document, '2019');
+		
+		return $document;
+	}
+}
diff --git a/tests/example_output/extension_members_everywhere/extension_members_everywhere.json b/tests/example_output/extension_members_everywhere/extension_members_everywhere.json
new file mode 100644
index 0000000..f41faa3
--- /dev/null
+++ b/tests/example_output/extension_members_everywhere/extension_members_everywhere.json
@@ -0,0 +1,89 @@
+{
+	"everywhere:key": "/key",
+	"jsonapi": {
+		"everywhere:key": "/jsonapi/key",
+		"version": "1.1",
+		"meta": {
+			"everywhere:key": "/jsonapi/meta/key"
+		}
+	},
+	"links": {
+		"everywhere:key": "/links/key",
+		"foo": {
+			"everywhere:key": "/links/foo/key",
+			"href": "https://jsonapi.org",
+			"meta": {
+				"everywhere:key": "/links/foo/meta/key"
+			}
+		}
+	},
+	"meta": {
+		"everywhere:key": "/meta/key"
+	},
+	"data": {
+		"type": "user",
+		"id": "42",
+		"everywhere:key": "/data/key",
+		"meta": {
+			"everywhere:key": "/data/meta/key"
+		},
+		"attributes": {
+			"everywhere:key": "/data/attributes/key"
+		},
+		"relationships": {
+			"everywhere:key": "/data/relationships/key",
+			"foo": {
+				"everywhere:key": "/data/relationships/foo/key",
+				"links": {
+					"everywhere:key": "/data/relationships/foo/links/key"
+				},
+				"data": {
+					"type": "user",
+					"id": "1"
+				},
+				"meta": {
+					"everywhere:key": "/data/relationships/foo/meta/key"
+				}
+			},
+			"bar": {
+				"everywhere:key": "/data/relationships/bar/key",
+				"data": {
+					"type": "user",
+					"id": "2",
+					"everywhere:key": "/data/relationships/bar/data/key",
+					"meta": {
+						"everywhere:key": "/data/relationships/bar/data/meta/key"
+					}
+				}
+			}
+		},
+		"links": {
+			"everywhere:key": "/data/links/key",
+			"foo": {
+				"everywhere:key": "/data/links/foo/key",
+				"href": "https://jsonapi.org",
+				"meta": {
+					"everywhere:key": "/data/links/foo/meta/key"
+				}
+			}
+		}
+	},
+	"included": [
+		{
+			"type": "user",
+			"id": "1",
+			"everywhere:key": "/included/0/key",
+			"attributes": {
+				"everywhere:key": "/included/0/attributes/key"
+			}
+		},
+		{
+			"type": "user",
+			"id": "3",
+			"everywhere:key": "/included/1/key",
+			"relationships": {
+				"everywhere:key": "/included/1/relationships/key"
+			}
+		}
+	]
+}
diff --git a/tests/example_output/extension_members_everywhere/extension_members_everywhere.php b/tests/example_output/extension_members_everywhere/extension_members_everywhere.php
new file mode 100644
index 0000000..f7747f2
--- /dev/null
+++ b/tests/example_output/extension_members_everywhere/extension_members_everywhere.php
@@ -0,0 +1,173 @@
+<?php
+
+namespace alsvanzelf\jsonapiTests\example_output\extension_members_everywhere;
+
+use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapi\objects\AttributesObject;
+use alsvanzelf\jsonapi\objects\JsonapiObject;
+use alsvanzelf\jsonapi\objects\LinkObject;
+use alsvanzelf\jsonapi\objects\LinksObject;
+use alsvanzelf\jsonapi\objects\MetaObject;
+use alsvanzelf\jsonapi\objects\RelationshipObject;
+use alsvanzelf\jsonapi\objects\RelationshipsObject;
+use alsvanzelf\jsonapi\objects\ResourceIdentifierObject;
+use alsvanzelf\jsonapi\objects\ResourceObject;
+use alsvanzelf\jsonapiTests\example_output\ExampleEverywhereExtension;
+
+class extension_members_everywhere {
+	public static function createJsonapiDocument() {
+		$extension = new ExampleEverywhereExtension();
+		
+		$document = new ResourceDocument('user', 42);
+		$document->applyExtension($extension);
+		
+		/**
+		 * root
+		 */
+		
+		$document->addExtensionMember($extension, 'key', '/key');
+		
+		/**
+		 * jsonapi
+		 */
+		
+		$metaObject = new MetaObject();
+		$metaObject->addExtensionMember($extension, 'key', '/jsonapi/meta/key');
+		
+		$jsonapiObject = new JsonapiObject();
+		$jsonapiObject->addExtensionMember($extension, 'key', '/jsonapi/key');
+		$jsonapiObject->setMetaObject($metaObject);
+		$document->setJsonapiObject($jsonapiObject);
+		
+		/**
+		 * links
+		 */
+		
+		$metaObject = new MetaObject();
+		$metaObject->addExtensionMember($extension, 'key', '/links/foo/meta/key');
+		
+		$linkObject = new LinkObject('https://jsonapi.org');
+		$linkObject->addExtensionMember($extension, 'key', '/links/foo/key');
+		$linkObject->setMetaObject($metaObject);
+		
+		$linksObject = new LinksObject();
+		$linksObject->addExtensionMember($extension, 'key', '/links/key');
+		$linksObject->addLinkObject('foo', $linkObject);
+		$document->setLinksObject($linksObject);
+		
+		/**
+		 * meta
+		 */
+		
+		$metaObject = new MetaObject();
+		$metaObject->addExtensionMember($extension, 'key', '/meta/key');
+		$document->setMetaObject($metaObject);
+		
+		/**
+		 * resource
+		 */
+		
+		/**
+		 * resource - relationships
+		 * 
+		 * @todo make it work to have extension members in both the identifier and the resource parts
+		 *       e.g. it is missing in the data of the first relationship (`data.relationships.foo.data.key`)
+		 *       whereas it does appear in the second relationship (`data.relationships.bar.data.key`)
+		 * @see https://github.com/json-api/json-api/issues/1367
+		 */
+		
+		$relationshipsObject = new RelationshipsObject();
+		$relationshipsObject->addExtensionMember($extension, 'key', '/data/relationships/key');
+		
+		$attributesObject = new AttributesObject();
+		$attributesObject->addExtensionMember($extension, 'key', '/included/0/attributes/key');
+		
+		$resourceObject = new ResourceObject('user', 1);
+		$resourceObject->addExtensionMember($extension, 'key', '/included/0/key');
+		$resourceObject->setAttributesObject($attributesObject);
+		
+		$linksObject = new LinksObject();
+		$linksObject->addExtensionMember($extension, 'key', '/data/relationships/foo/links/key');
+		
+		$metaObject = new MetaObject();
+		$metaObject->addExtensionMember($extension, 'key', '/data/relationships/foo/meta/key');
+		
+		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject->addExtensionMember($extension, 'key', '/data/relationships/foo/key');
+		$relationshipObject->setResource($resourceObject);
+		$relationshipObject->setLinksObject($linksObject);
+		$relationshipObject->setMetaObject($metaObject);
+		
+		$relationshipsObject->addRelationshipObject('foo', $relationshipObject);
+		
+		$metaObject = new MetaObject();
+		$metaObject->addExtensionMember($extension, 'key', '/data/relationships/bar/data/meta/key');
+		
+		$resourceIdentifierObject = new ResourceIdentifierObject('user', 2);
+		$resourceIdentifierObject->addExtensionMember($extension, 'key', '/data/relationships/bar/data/key');
+		$resourceIdentifierObject->setMetaObject($metaObject);
+		
+		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		$relationshipObject->addExtensionMember($extension, 'key', '/data/relationships/bar/key');
+		$relationshipObject->setResource($resourceIdentifierObject);
+		
+		$relationshipsObject->addRelationshipObject('bar', $relationshipObject);
+		
+		/**
+		 * resource - attributes
+		 */
+		
+		$attributesObject = new AttributesObject();
+		$attributesObject->addExtensionMember($extension, 'key', '/data/attributes/key');
+		
+		/**
+		 * resource - links
+		 */
+		
+		$metaObject = new MetaObject();
+		$metaObject->addExtensionMember($extension, 'key', '/data/links/foo/meta/key');
+		
+		$linkObject = new LinkObject('https://jsonapi.org');
+		$linkObject->addExtensionMember($extension, 'key', '/data/links/foo/key');
+		$linkObject->setMetaObject($metaObject);
+		
+		$linksObject = new LinksObject();
+		$linksObject->addExtensionMember($extension, 'key', '/data/links/key');
+		$linksObject->addLinkObject('foo', $linkObject);
+		
+		/**
+		 * resource - meta
+		 */
+		
+		$metaObject = new MetaObject();
+		$metaObject->addExtensionMember($extension, 'key', '/data/meta/key');
+		
+		/**
+		 * resource - resource
+		 */
+		
+		$resourceObject = new ResourceObject('user', 42);
+		$resourceObject->addExtensionMember($extension, 'key', '/data/key');
+		$resourceObject->setAttributesObject($attributesObject);
+		$resourceObject->setLinksObject($linksObject);
+		$resourceObject->setMetaObject($metaObject);
+		$resourceObject->setRelationshipsObject($relationshipsObject);
+		
+		$document->setPrimaryResource($resourceObject);
+		
+		/**
+		 * included
+		 */
+		
+		$relationshipsObject = new RelationshipsObject();
+		$relationshipsObject->addExtensionMember($extension, 'key', '/included/1/relationships/key');
+		
+		$resourceObject = new ResourceObject('user', 3);
+		$resourceObject->addExtensionMember($extension, 'key', '/included/1/key');
+		$resourceObject->setRelationshipsObject($relationshipsObject);
+		
+		$document->addIncludedResourceObject($resourceObject);
+		
+		return $document;
+	}
+}
diff --git a/tests/example_output/profile/profile.json b/tests/example_output/profile/profile.json
new file mode 100644
index 0000000..49d5808
--- /dev/null
+++ b/tests/example_output/profile/profile.json
@@ -0,0 +1,18 @@
+{
+	"jsonapi": {
+		"version": "1.1",
+		"profile": [
+			"https://jsonapi.org/recommendations/#authoring-profiles"
+		]
+	},
+	"data": {
+		"type": "user",
+		"id": "42",
+		"attributes": {
+			"timestamps": {
+				"created": "2019-01-01T00:00:00+0000",
+				"updated": "2021-01-01T00:00:00+0000"
+			}
+		}
+	}
+}
diff --git a/tests/example_output/profile/profile.php b/tests/example_output/profile/profile.php
new file mode 100644
index 0000000..16deba8
--- /dev/null
+++ b/tests/example_output/profile/profile.php
@@ -0,0 +1,19 @@
+<?php
+
+namespace alsvanzelf\jsonapiTests\example_output\profile;
+
+use alsvanzelf\jsonapi\ResourceDocument;
+use alsvanzelf\jsonapiTests\example_output\ExampleTimestampsProfile;
+
+class profile {
+	public static function createJsonapiDocument() {
+		$profile = new ExampleTimestampsProfile();
+		
+		$document = new ResourceDocument('user', 42);
+		$document->applyProfile($profile);
+		
+		$profile->setTimestamps($document, new \DateTime('2019-01-01T00:00:00+0000'), new \DateTime('2021-01-01T00:00:00+0000'));
+		
+		return $document;
+	}
+}
diff --git a/tests/extensions/AtomicOperationsDocumentTest.php b/tests/extensions/AtomicOperationsDocumentTest.php
new file mode 100644
index 0000000..523ca22
--- /dev/null
+++ b/tests/extensions/AtomicOperationsDocumentTest.php
@@ -0,0 +1,51 @@
+<?php
+
+namespace alsvanzelf\jsonapiTests\extensions;
+
+use alsvanzelf\jsonapi\extensions\AtomicOperationsDocument;
+use alsvanzelf\jsonapi\extensions\AtomicOperationsExtension;
+use alsvanzelf\jsonapi\objects\ResourceObject;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * @group Extensions
+ */
+class AtomicOperationsDocumentTest extends TestCase {
+	public function testSetResults_HappyPath() {
+		$document = new AtomicOperationsDocument();
+		
+		$resource1 = new ResourceObject('person', 1);
+		$resource2 = new ResourceObject('person', 2);
+		$resource3 = new ResourceObject('person', 3);
+		$resource1->add('name', 'Ford');
+		$resource2->add('name', 'Arthur');
+		$resource3->add('name', 'Zaphod');
+		$document->addResults($resource1, $resource2, $resource3);
+		
+		$array = $document->toArray();
+		
+		$this->assertArrayHasKey('jsonapi', $array);
+		$this->assertArrayHasKey('ext', $array['jsonapi']);
+		$this->assertCount(1, $array['jsonapi']['ext']);
+		$this->assertSame((new AtomicOperationsExtension())->getOfficialLink(), $array['jsonapi']['ext'][0]);
+		
+		$this->assertArrayHasKey('atomic:results', $array);
+		$this->assertCount(3, $array['atomic:results']);
+		$this->assertSame(['data' => $resource1->toArray()], $array['atomic:results'][0]);
+		$this->assertSame(['data' => $resource2->toArray()], $array['atomic:results'][1]);
+		$this->assertSame(['data' => $resource3->toArray()], $array['atomic:results'][2]);
+	}
+	
+	public function testSetResults_EmptySuccessResults() {
+		$document = new AtomicOperationsDocument();
+		$array    = $document->toArray();
+		
+		$this->assertArrayHasKey('jsonapi', $array);
+		$this->assertArrayHasKey('ext', $array['jsonapi']);
+		$this->assertCount(1, $array['jsonapi']['ext']);
+		$this->assertSame((new AtomicOperationsExtension())->getOfficialLink(), $array['jsonapi']['ext'][0]);
+		
+		$this->assertArrayHasKey('atomic:results', $array);
+		$this->assertCount(0, $array['atomic:results']);
+	}
+}
diff --git a/tests/extensions/TestExtension.php b/tests/extensions/TestExtension.php
new file mode 100644
index 0000000..9b57f9e
--- /dev/null
+++ b/tests/extensions/TestExtension.php
@@ -0,0 +1,26 @@
+<?php
+
+namespace alsvanzelf\jsonapiTests\extensions;
+
+use alsvanzelf\jsonapi\interfaces\ExtensionInterface;
+
+class TestExtension implements ExtensionInterface {
+	private $namespace;
+	private $officialLink;
+	
+	public function setNamespace($namespace) {
+		$this->namespace = $namespace;
+	}
+	
+	public function setOfficialLink($officialLink) {
+		$this->officialLink = $officialLink;
+	}
+	
+	public function getNamespace() {
+		return $this->namespace;
+	}
+	
+	public function getOfficialLink() {
+		return $this->officialLink;
+	}
+}
diff --git a/tests/helpers/ExtensionMemberManagerTest.php b/tests/helpers/ExtensionMemberManagerTest.php
new file mode 100644
index 0000000..92616bb
--- /dev/null
+++ b/tests/helpers/ExtensionMemberManagerTest.php
@@ -0,0 +1,70 @@
+<?php
+
+namespace alsvanzelf\jsonapiTests;
+
+use alsvanzelf\jsonapi\exceptions\InputException;
+use alsvanzelf\jsonapiTests\helpers\TestableNonTraitExtensionMemberManager as ExtensionMemberManager;
+use alsvanzelf\jsonapiTests\extensions\TestExtension;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * @group Extensions
+ */
+class ExtensionMemberManagerTest extends TestCase {
+	public function testAddExtensionMember_HappyPath() {
+		$helper    = new ExtensionMemberManager();
+		$extension = new TestExtension();
+		$extension->setNamespace('test');
+		
+		$this->assertFalse($helper->hasExtensionMembers());
+		$this->assertSame([], $helper->getExtensionMembers());
+		
+		$helper->addExtensionMember($extension, 'foo', 'bar');
+		
+		$array = $helper->getExtensionMembers();
+		
+		$this->assertTrue($helper->hasExtensionMembers());
+		$this->assertCount(1, $array);
+		$this->assertArrayHasKey('test:foo', $array);
+		$this->assertSame('bar', $array['test:foo']);
+	}
+	
+	public function testAddExtensionMember_WithNamespacePrefixed() {
+		$helper    = new ExtensionMemberManager();
+		$extension = new TestExtension();
+		$extension->setNamespace('test');
+		
+		$helper->addExtensionMember($extension, 'test:foo', 'bar');
+		
+		$array = $helper->getExtensionMembers();
+		
+		$this->assertArrayHasKey('test:foo', $array);
+	}
+	
+	public function testAddExtensionMember_WithObjectValue() {
+		$helper    = new ExtensionMemberManager();
+		$extension = new TestExtension();
+		$extension->setNamespace('test');
+		
+		$object = new \stdClass();
+		$object->bar = 'baz';
+		
+		$helper->addExtensionMember($extension, 'foo', $object);
+		
+		$array = $helper->getExtensionMembers();
+		
+		$this->assertArrayHasKey('test:foo', $array);
+		$this->assertArrayHasKey('bar', $array['test:foo']);
+		$this->assertSame('baz', $array['test:foo']['bar']);
+	}
+	
+	public function testAddExtensionMember_InvalidNamespaceOrCharacter() {
+		$helper    = new ExtensionMemberManager();
+		$extension = new TestExtension();
+		$extension->setNamespace('test');
+		
+		$this->expectException(InputException::class);
+		
+		$helper->addExtensionMember($extension, 'foo:bar', 'baz');
+	}
+}
diff --git a/tests/helpers/TestableNonTraitExtensionMemberManager.php b/tests/helpers/TestableNonTraitExtensionMemberManager.php
new file mode 100644
index 0000000..516f103
--- /dev/null
+++ b/tests/helpers/TestableNonTraitExtensionMemberManager.php
@@ -0,0 +1,12 @@
+<?php
+
+namespace alsvanzelf\jsonapiTests\helpers;
+
+use alsvanzelf\jsonapi\helpers\ExtensionMemberManager;
+
+/**
+ * using ExtensionMemberManager to make it non-trait to test against it
+ */
+class TestableNonTraitExtensionMemberManager {
+	use ExtensionMemberManager;
+}
diff --git a/tests/objects/ErrorObjectTest.php b/tests/objects/ErrorObjectTest.php
index 2c41bb5..7046e31 100644
--- a/tests/objects/ErrorObjectTest.php
+++ b/tests/objects/ErrorObjectTest.php
@@ -4,6 +4,7 @@
 
 use alsvanzelf\jsonapi\exceptions\InputException;
 use alsvanzelf\jsonapi\objects\ErrorObject;
+use alsvanzelf\jsonapiTests\extensions\TestExtension;
 use PHPUnit\Framework\TestCase;
 
 class ErrorObjectTest extends TestCase {
@@ -152,6 +153,28 @@ public function testIsEmpty_All() {
 		$errorObject = new ErrorObject();
 		$errorObject->addAtMember('context', 'test');
 		$this->assertFalse($errorObject->isEmpty());
+		
+		$errorObject = new ErrorObject();
+		$errorObject->addExtensionMember(new TestExtension(), 'foo', 'bar');
+		$this->assertFalse($errorObject->isEmpty());
+	}
+	
+	/**
+	 * @group Extensions
+	 */
+	public function testToArray_WithExtensionMembers() {
+		$errorObject = new ErrorObject();
+		$extension   = new TestExtension();
+		$extension->setNamespace('test');
+		
+		$this->assertSame([], $errorObject->toArray());
+		
+		$errorObject->addExtensionMember($extension, 'foo', 'bar');
+		
+		$array = $errorObject->toArray();
+		
+		$this->assertArrayHasKey('test:foo', $array);
+		$this->assertSame('bar', $array['test:foo']);
 	}
 }
 
diff --git a/tests/objects/JsonapiObjectTest.php b/tests/objects/JsonapiObjectTest.php
index 0807a19..882c708 100644
--- a/tests/objects/JsonapiObjectTest.php
+++ b/tests/objects/JsonapiObjectTest.php
@@ -3,6 +3,8 @@
 namespace alsvanzelf\jsonapiTests\objects;
 
 use alsvanzelf\jsonapi\objects\JsonapiObject;
+use alsvanzelf\jsonapiTests\extensions\TestExtension;
+use alsvanzelf\jsonapiTests\profiles\TestProfile;
 use PHPUnit\Framework\TestCase;
 
 class JsonapiObjectTest extends TestCase {
@@ -31,4 +33,43 @@ public function testIsEmpty_WithAtMembers() {
 		
 		$this->assertFalse($jsonapiObject->isEmpty());
 	}
+	
+	/**
+	 * @group Extensions
+	 */
+	public function testIsEmpty_WithExtensionLink() {
+		$jsonapiObject = new JsonapiObject($version=null);
+		
+		$this->assertTrue($jsonapiObject->isEmpty());
+		
+		$jsonapiObject->addExtension(new TestExtension());
+		
+		$this->assertFalse($jsonapiObject->isEmpty());
+	}
+	
+	/**
+	 * @group Profiles
+	 */
+	public function testIsEmpty_WithProfileLink() {
+		$jsonapiObject = new JsonapiObject($version=null);
+		
+		$this->assertTrue($jsonapiObject->isEmpty());
+		
+		$jsonapiObject->addProfile(new TestProfile());
+		
+		$this->assertFalse($jsonapiObject->isEmpty());
+	}
+	
+	/**
+	 * @group Extensions
+	 */
+	public function testIsEmpty_WithExtensionMembers() {
+		$jsonapiObject = new JsonapiObject($version=null);
+		
+		$this->assertTrue($jsonapiObject->isEmpty());
+		
+		$jsonapiObject->addExtensionMember(new TestExtension(), 'foo', 'bar');
+		
+		$this->assertFalse($jsonapiObject->isEmpty());
+	}
 }
diff --git a/tests/objects/LinkObjectTest.php b/tests/objects/LinkObjectTest.php
index ad41068..fd356c9 100644
--- a/tests/objects/LinkObjectTest.php
+++ b/tests/objects/LinkObjectTest.php
@@ -3,6 +3,7 @@
 namespace alsvanzelf\jsonapiTests\objects;
 
 use alsvanzelf\jsonapi\objects\LinkObject;
+use alsvanzelf\jsonapiTests\extensions\TestExtension;
 use PHPUnit\Framework\TestCase;
 
 class LinkObjectTest extends TestCase {
@@ -151,4 +152,17 @@ public function testIsEmpty_WithAtMembers() {
 		
 		$this->assertFalse($linkObject->isEmpty());
 	}
+	
+	/**
+	 * @group Extensions
+	 */
+	public function testIsEmpty_WithExtensionMembers() {
+		$linkObject = new LinkObject();
+		
+		$this->assertTrue($linkObject->isEmpty());
+		
+		$linkObject->addExtensionMember(new TestExtension(), 'foo', 'bar');
+		
+		$this->assertFalse($linkObject->isEmpty());
+	}
 }
diff --git a/tests/objects/MetaObjectTest.php b/tests/objects/MetaObjectTest.php
index 5014060..fc4cb6a 100644
--- a/tests/objects/MetaObjectTest.php
+++ b/tests/objects/MetaObjectTest.php
@@ -3,6 +3,7 @@
 namespace alsvanzelf\jsonapiTests\objects;
 
 use alsvanzelf\jsonapi\objects\MetaObject;
+use alsvanzelf\jsonapiTests\extensions\TestExtension;
 use PHPUnit\Framework\TestCase;
 
 class MetaObjectTest extends TestCase {
@@ -18,4 +19,27 @@ public function testFromObject_HappyPath() {
 		$this->assertArrayHasKey('foo', $array);
 		$this->assertSame('bar', $array['foo']);
 	}
+	
+	public function testIsEmpty_WithAtMembers() {
+		$metaObject = new MetaObject();
+		
+		$this->assertTrue($metaObject->isEmpty());
+		
+		$metaObject->addAtMember('context', 'test');
+		
+		$this->assertFalse($metaObject->isEmpty());
+	}
+	
+	/**
+	 * @group Extensions
+	 */
+	public function testIsEmpty_WithExtensionMembers() {
+		$metaObject = new MetaObject();
+		
+		$this->assertTrue($metaObject->isEmpty());
+		
+		$metaObject->addExtensionMember(new TestExtension(), 'foo', 'bar');
+		
+		$this->assertFalse($metaObject->isEmpty());
+	}
 }
diff --git a/tests/objects/RelationshipObjectTest.php b/tests/objects/RelationshipObjectTest.php
index 346480a..e5a99fb 100644
--- a/tests/objects/RelationshipObjectTest.php
+++ b/tests/objects/RelationshipObjectTest.php
@@ -9,6 +9,7 @@
 use alsvanzelf\jsonapi\objects\RelationshipObject;
 use alsvanzelf\jsonapi\objects\ResourceIdentifierObject;
 use alsvanzelf\jsonapi\objects\ResourceObject;
+use alsvanzelf\jsonapiTests\extensions\TestExtension;
 use PHPUnit\Framework\TestCase;
 
 class RelationshipObjectTest extends TestCase {
@@ -315,6 +316,19 @@ public function testIsEmpty_WithAtMembers() {
 		$this->assertFalse($relationshipObject->isEmpty());
 	}
 	
+	/**
+	 * @group Extensions
+	 */
+	public function testIsEmpty_WithExtensionMembers() {
+		$relationshipObject = new RelationshipObject(RelationshipObject::TO_ONE);
+		
+		$this->assertTrue($relationshipObject->isEmpty());
+		
+		$relationshipObject->addExtensionMember(new TestExtension(), 'foo', 'bar');
+		
+		$this->assertFalse($relationshipObject->isEmpty());
+	}
+	
 	private function validateToOneRelationshipArray(array $array) {
 		$this->assertNotEmpty($array);
 		$this->assertArrayHasKey('data', $array);
diff --git a/tests/objects/ResourceIdentifierObjectTest.php b/tests/objects/ResourceIdentifierObjectTest.php
index edee0bd..b7eb1ea 100644
--- a/tests/objects/ResourceIdentifierObjectTest.php
+++ b/tests/objects/ResourceIdentifierObjectTest.php
@@ -5,6 +5,7 @@
 use alsvanzelf\jsonapi\exceptions\Exception;
 use alsvanzelf\jsonapi\exceptions\DuplicateException;
 use alsvanzelf\jsonapi\objects\ResourceIdentifierObject;
+use alsvanzelf\jsonapiTests\extensions\TestExtension;
 use PHPUnit\Framework\TestCase;
 
 class ResourceIdentifierObjectTest extends TestCase {
@@ -174,4 +175,17 @@ public function testIsEmpty_WithAtMembers() {
 		
 		$this->assertFalse($resourceIdentifierObject->isEmpty());
 	}
+	
+	/**
+	 * @group Extensions
+	 */
+	public function testIsEmpty_WithExtensionMembers() {
+		$resourceIdentifierObject = new ResourceIdentifierObject();
+		
+		$this->assertTrue($resourceIdentifierObject->isEmpty());
+		
+		$resourceIdentifierObject->addExtensionMember(new TestExtension(), 'foo', 'bar');
+		
+		$this->assertFalse($resourceIdentifierObject->isEmpty());
+	}
 }
diff --git a/tests/profiles/CursorPaginationProfileTest.php b/tests/profiles/CursorPaginationProfileTest.php
index cd40ec4..4fb3db8 100644
--- a/tests/profiles/CursorPaginationProfileTest.php
+++ b/tests/profiles/CursorPaginationProfileTest.php
@@ -20,10 +20,16 @@ public function testSetLinks_HappyPath() {
 		$firstCursor      = 'bar';
 		$lastCursor       = 'foo';
 		
+		$collection->applyProfile($profile);
 		$profile->setLinks($collection, $baseOrCurrentUrl, $firstCursor, $lastCursor);
 		
 		$array = $collection->toArray();
 		
+		$this->assertArrayHasKey('jsonapi', $array);
+		$this->assertArrayHasKey('profile', $array['jsonapi']);
+		$this->assertCount(1, $array['jsonapi']['profile']);
+		$this->assertSame($profile->getOfficialLink(), $array['jsonapi']['profile'][0]);
+		
 		$this->assertArrayHasKey('links', $array);
 		$this->assertCount(2, $array['links']);
 		$this->assertArrayHasKey('prev', $array['links']);
@@ -37,6 +43,7 @@ public function testSetLinks_HappyPath() {
 	public function test_WithRelationship() {
 		$profile  = new CursorPaginationProfile();
 		$document = new ResourceDocument('test', 1);
+		$document->applyProfile($profile);
 		
 		$person1  = new ResourceObject('person', 1);
 		$person2  = new ResourceObject('person', 2);
@@ -59,6 +66,11 @@ public function test_WithRelationship() {
 		
 		$array = $document->toArray();
 		
+		$this->assertArrayHasKey('jsonapi', $array);
+		$this->assertArrayHasKey('profile', $array['jsonapi']);
+		$this->assertCount(1, $array['jsonapi']['profile']);
+		$this->assertSame($profile->getOfficialLink(), $array['jsonapi']['profile'][0]);
+		
 		$this->assertArrayHasKey('data', $array);
 		$this->assertArrayHasKey('relationships', $array['data']);
 		$this->assertArrayHasKey('people', $array['data']['relationships']);
@@ -85,10 +97,16 @@ public function testSetLinksFirstPage_HappyPath() {
 		$baseOrCurrentUrl = '/people?page[size]=10';
 		$lastCursor       = 'foo';
 		
+		$collection->applyProfile($profile);
 		$profile->setLinksFirstPage($collection, $baseOrCurrentUrl, $lastCursor);
 		
 		$array = $collection->toArray();
 		
+		$this->assertArrayHasKey('jsonapi', $array);
+		$this->assertArrayHasKey('profile', $array['jsonapi']);
+		$this->assertCount(1, $array['jsonapi']['profile']);
+		$this->assertSame($profile->getOfficialLink(), $array['jsonapi']['profile'][0]);
+		
 		$this->assertArrayHasKey('links', $array);
 		$this->assertCount(2, $array['links']);
 		$this->assertArrayHasKey('prev', $array['links']);
@@ -104,10 +122,16 @@ public function testSetLinksLastPage_HappyPath() {
 		$baseOrCurrentUrl = '/people?page[size]=10';
 		$firstCursor      = 'bar';
 		
+		$collection->applyProfile($profile);
 		$profile->setLinksLastPage($collection, $baseOrCurrentUrl, $firstCursor);
 		
 		$array = $collection->toArray();
 		
+		$this->assertArrayHasKey('jsonapi', $array);
+		$this->assertArrayHasKey('profile', $array['jsonapi']);
+		$this->assertCount(1, $array['jsonapi']['profile']);
+		$this->assertSame($profile->getOfficialLink(), $array['jsonapi']['profile'][0]);
+		
 		$this->assertArrayHasKey('links', $array);
 		$this->assertCount(2, $array['links']);
 		$this->assertArrayHasKey('prev', $array['links']);
@@ -121,10 +145,16 @@ public function testSetCursor() {
 		$profile          = new CursorPaginationProfile();
 		$resourceDocument = new ResourceDocument('user', 42);
 		
+		$resourceDocument->applyProfile($profile);
 		$profile->setCursor($resourceDocument, 'foo');
 		
 		$array = $resourceDocument->toArray();
 		
+		$this->assertArrayHasKey('jsonapi', $array);
+		$this->assertArrayHasKey('profile', $array['jsonapi']);
+		$this->assertCount(1, $array['jsonapi']['profile']);
+		$this->assertSame($profile->getOfficialLink(), $array['jsonapi']['profile'][0]);
+		
 		$this->assertArrayHasKey('data', $array);
 		$this->assertArrayHasKey('meta', $array['data']);
 		$this->assertArrayHasKey('page', $array['data']['meta']);
@@ -136,10 +166,16 @@ public function testSetPaginationLinkObjectsExplicitlyEmpty_HapptPath() {
 		$profile    = new CursorPaginationProfile();
 		$collection = new CollectionDocument();
 		
+		$collection->applyProfile($profile);
 		$profile->setPaginationLinkObjectsExplicitlyEmpty($collection);
 		
 		$array = $collection->toArray();
 		
+		$this->assertArrayHasKey('jsonapi', $array);
+		$this->assertArrayHasKey('profile', $array['jsonapi']);
+		$this->assertCount(1, $array['jsonapi']['profile']);
+		$this->assertSame($profile->getOfficialLink(), $array['jsonapi']['profile'][0]);
+		
 		$this->assertArrayHasKey('links', $array);
 		$this->assertCount(2, $array['links']);
 		$this->assertArrayHasKey('prev', $array['links']);
@@ -155,10 +191,16 @@ public function testSetPaginationMeta() {
 		$bestGuessTotal   = 100;
 		$rangeIsTruncated = true;
 		
+		$collection->applyProfile($profile);
 		$profile->setPaginationMeta($collection, $exactTotal, $bestGuessTotal, $rangeIsTruncated);
 		
 		$array = $collection->toArray();
 		
+		$this->assertArrayHasKey('jsonapi', $array);
+		$this->assertArrayHasKey('profile', $array['jsonapi']);
+		$this->assertCount(1, $array['jsonapi']['profile']);
+		$this->assertSame($profile->getOfficialLink(), $array['jsonapi']['profile'][0]);
+		
 		$this->assertArrayHasKey('meta', $array);
 		$this->assertArrayHasKey('page', $array['meta']);
 		$this->assertArrayHasKey('total', $array['meta']['page']);