This chapter presents the fundamentals of Medium access control (MAC) protocols and explains the specific requirements and problems these protocols have to withstand for Wireless Sensor Networks. MAC layer is a part of the data link layer in the Open System Internet reference model. The most important performance measurements for MAC protocols are throughput, fairness, stability, low access delay, low transmission delay, low overhead, and less energy consumption. The operation and performance of MAC protocols is heavily influenced by the properties of the physical layer. The hidden node problem occurs in Carrier Sense Multiple Access protocols in which a node senses the medium before the start of transmission. The fixed assignment class of protocols is collision free, and all the available resources are divided between the nodes such that each node can use its resources exclusively. The transmitter sends a beacon signal during the wake-up period periodically without the performance of carrier sensing.