The cognitive radio network (CRN) has been paid more attentions as it can enable the device to dynamically access unused spectrum without interference to primary users (PUs). The medium access control (MAC) protocol of CRN, has being intensively researched for its key function position. This paper reviews the demand of MAC for CRN, the state-of-the-art spectrum sensing technology, and a detailed classification of the MAC protocols. In this overview, the characteristic features, advantages, and the limiting factors of the existing CRN MAC protocols are thoroughly investigated for both underlay-based and overlay-based networks. Several challenges and future research directions are also presented in this paper.