The interfaces of RESTful, hypertext-driven applications consist of reusable components such as Internet media types and link relation types. This document defines a simple standard that application designers can use to describe the interface of their application in a structured way so that other parties can develop interoperable clients and servers or reuse the components in their own applications. Note to Readers This Internet-Draft should be discussed on the Thing-to-Thing Research Group (T2TRG) mailing list <t2trg@irtf.org>.