This SpringerBrief presents recent advances in the cognitive MAC designs for opportunistic spectrum access (OSA) networks. It covers the basic MAC functionalities and MAC enhancements of IEEE 802.11. Later chapters discuss the existing MAC protocols for OSA and classify them based on characteristic features. The authors provide new research in adaptive carrier sensing-based MAC designs tailored for OSA, which optimize spectrum utilization and ensure a peaceful coexistence of licensed and unlicensed systems. Analytically devised via optimization and game-theoretic approaches, these adaptive MAC designs are shown to effectively reduce collisions between both primary and secondary network users. Researchers and professionals working in wireless communications and networks will find this content valuable. This brief is also a useful study guide for advanced-level students in computer science and electrical engineering.