This document introduces HYDRA, a lightweight vocabulary that allows the creation of hypermedia-driven web APIs. HYDRA extends JSON-LD to provide semantics that enable servers to advertise valid state transitions to clients. This allows generic clients to understand operations on APIs and navigate through them using hyperlinks. HYDRA aims to address issues with existing RESTful APIs, such as clients needing to be re-written when APIs change, by making APIs self-descriptive through linked data.