Almost 12 years ago, Roy Fielding introduced Representational State Transfer (REST) in his dissertation on Architectural Styles and the Design of Network-based Software Architectures. Since then, APIs adopting the REST architectural style (so-called "RESTful" APIs) have gradually increased in popularity. Nonetheless, a key constraint that Fielding proposed has yet to be adopted as a mainstream feature of RESTful APIs. This feature is known as Hypermedia as the Engine of Application State, or HATEOAS. APIs that enforce this constraint are referred to as HATEOAS-compliant APIs. More generally, APIs that adopt the characteristics of HATEOAS are called "Hypermedia APIs."