Ad hoc and multi-hop networks will probably be a part of the fourth generation of wireless networks, which will integrate networks of several sizes and capacities with heterogeneous coverage: cellular networks (3G), WLAN hot spots, wireless personal area networks (WPAN) and wireless body area networks (WBAN). In this context, MAC protocols play a deciding role for a high utilization of the wireless channel. In this paper, several issues of the MAC layer and concepts for the definition of a new MAC protocol are presented. These concepts include synchronization, multi-user diversity, and multi-packet reception. It is shown that all these techniques can drastically increase the capacity of the MAC layer for multi-hop networks.