What is HATEOAS and how is it implemented?

HATEOAS (Hypermedia as the Engine of Application State) is the highest level of REST maturity. It means API responses include hyperlinks that tell the client what actions are available next, removing the need for clients to have hardcoded knowledge of URL structures. Example: a response to GET /orders/42 might include links like { "self": "/orders/42", "cancel": "/orders/42/cancel", "payment": "/orders/42/payment" } — the client follows these links rather than constructing URLs. The HAL (Hypertext Application Language) and JSON:API formats standardize how links are embedded. While HATEOAS is the theoretically correct REST approach, it adds complexity and is rarely implemented fully in practice — most APIs are pragmatic and treat HATEOAS as optional.