  Recently, significant research attention has been devoted to the study of
reconfigurable intelligent surfaces (RISs), which are capable of reconfiguring
the wireless propagation environment by exploiting the unique properties of
metamaterials-based integrated large arrays of inexpensive antennas. Existing
research demonstrates that RISs significantly improve the physical layer
performance, including the wireless coverage, achievable data rate and energy
efficiency. However, the medium access control (MAC) of multiple users
accessing an RIS-enabled channel is still in its infancy, while many open
issues remain to be addressed. In this article, we present four typical
RIS-aided multi-user scenarios with special emphasis on the MAC schemes. We
then propose and elaborate upon centralized, distributed and hybrid
artificial-intelligence (AI)-assisted MAC architectures in RIS-aided multi-user
communications systems. Finally, we discuss some challenges, perspectives and
potential applications of RISs as they are related to MAC design.
