As wireless communication technology becomes increasingly available, multi-hop wireless networks such as wireless sensor networks and wireless mesh networks are receiving substantial attention from both academic and industrial efforts. However, traditional contention-based wireless MAC protocols such as IEEE 802.11 DCF are designed for single-hop wireless networks and do not perform well in multi-hop scenarios due to inefficiency in their medium reservation mechanisms. In this thesis, a new general mechanism, utilizing a new Pioneer (PION) scheduling frame, is introduced to improve the efficiency in wireless medium reservation for multi-hop wireless networks. With the help of limited routing information at the MAC layer, this new mechanism enables multiple stations along a delivery path to cooperate. In particular, the PION control frame can travel across multiple hops and make medium reservations along the route for the upcoming data frame transmissions. This cooperation across multiple hops at the MAC layer can therefore greatly improve the medium reservation efficiency. This thesis introduces the PION mechanism and illustrates its use through application in two different types of MAC protocols. First, in the energy-constrained sensor network environment, I develop a new a duty-cycle based MAC protocol, called RMAC. Comparing against S-MAC, a classic duty-cycle MAC protocol, RMAC shows significant performance improvement in both packet delivery latency and network throughput, with even better energy efficiency. Second, the new PION mechanism is then applied to the general case of multi-hop wireless networks in the design of a MAC protocol called EMAC. Simulation results under various scenarios and traffic loads have shown the potential of EMAC over IEEE 802.11, including throughput improvement and decrease in packet relaying time.