Medium access control (MAC), also known as multiple access control, is a sublayer of the data link layer and manages access to the medium. In underwater networks, MAC protocols orchestrate the access to the acoustic communication channel. Without MAC, collisions of unsolicited modem signals may greatly degrade the overall network performance. The basic MAC objective is to avoid collisions, but more generally MAC protocols deal with network throughput, latency, energy efficiency, scalability, and adaptability. Weights can be given to different MAC objectives, depending on application and requirements. MAC protocols can be subdivided in contention-free schemes and contention-based schemes.