In cognitive radio networks, methods to allow the efficient sharing of the radio spectrum without causing any harmful interference to the primary users and to solve the spectrum scarcity problem are necessary. Identifying the available spectrum resources through spectrum sensing, coordinating with other users for spectrum access, improving channel utility, reducing collision rates, and improving sensing overhead are the major factors in cognitive radio medium access control protocols. Efficient use of radio spectrum largely depends on accurate spectrum sensing, spectrum sharing, spectrum mobility, and spectrum decisions. Hence, cognitive radio technology can significantly solve the spectrum scarcity problem by exploiting radio spectrum unused by licensed users. However, many technical problems still need to be solved for the proper functioning of cognitive radio networks. The medium access control protocols for cognitive radio networks should provide a comprehensive description of common control channels, spectrum sensing, harmful interference to primary users, variation of the spectrum availability, infrastructure support, the need for time synchronization, and the number of radio transceivers. In this paper, we present an overview of current medium access control protocols. Moreover, the classifications of medium access control protocols with their relative advantages and disadvantages are thoroughly investigated. We also highlight and explore some important research issues and challenges that could drive future research in this area.