Hypermedia as the engine of application state (HATEOAS) is a constraint of the REST software architectural style that distinguishes it from other network architectural styles.

With HATEOAS, a client interacts with a network application whose application servers provide information dynamically through hypermedia. A REST client needs little to no prior knowledge about how to interact with an application or server beyond a generic understanding of hypermedia.

By contrast, clients and servers in Common Object Request Broker Architecture (CORBA) interact through a fixed interface shared through documentation or an interface description language (IDL).

The restrictions imposed by HATEOAS decouple client and server. This enables server functionality to evolve independently.

The term was coined in 2000 by Roy Fielding in his doctoral dissertation.

A user-agent makes an HTTP request to a REST API through an entry point URL. All subsequent requests the user-agent may make are discovered inside the response to each request. The media types used for these representations, and the link relations they may contain, are part of the API. The client transitions through application states by selecting from the links within a representation or by manipulating the representation in other ways afforded by its media type. In this way, RESTful interaction is driven by hypermedia, rather than out-of-band information.

For example, this GET request fetches an account resource, requesting details in a JSON representation:

The response is: