A well studied problem in the engineering of open MASs is to enable uniform interaction among heterogeneous agents. However, AOSE as a field has grown to recognize that a MAS consists of more than only agents and thus should be designed on multiple dimensions (including the environment, organization etc.). The problem of enabling interaction among heterogeneous entities across dimensions is either not considered, or it is addressed in an ad hoc and non-uniform manner. In this chapter, we introduce a novel approach to use hypermedia as a general mechanism to support uniform interaction in MASs. The core idea is that agents use hypermedia to discover at runtime (i) other entities in a MAS (e.g., other agents, tools, organizations) and (ii) the means to interact with those entities (e.g., interaction protocols, APIs). This reduces coupling and enhances the scalability and evolvability of the MAS. We present a demonstrator that supports these claims. We believe that a hypermedia-based mechanism for uniform interaction in MASs could provide a foundation for engineering worldwide MASs.