Energy is a critical resource in the design of Ad-hoc and multi-hop wireless networks, since wireless devices are often powered by batteries which provide a finite amount of energy. Hence, there is a need to reduce the energy demands of the components that make up a wireless network, ~d in particular the IEEE 802.11 network interface. Ad-hoc wireless networks are currently receiving a significant amount of interest. Some of this interest may be attributed to the distributed nature ofIEEE 802.11's DCF, which allows for instant deployment and routing of packets around nodes in ad-hoc multi-hop wireless networks. In the IEEE 802.11 DCF, the wireless channel needs t.o be shared efficiently among contending nodes, and considerable research efforts are being dedicated to improving the energy consumption in these networks. In this thesis several aspects of energy consumption and the efficient utilization of limited energy resources of IEEE 802.11-based communication systems are explored. The studies provide para!11eters that are used to determine the energy consumption of the network interface for various modes of operation, modulation schemes, .transmission rates and different radio link qualities. A new metric, the energy consumed to transmit one payload bit successfully, is employed to determine meaningful power consumption. Three new approaches for conserving the energy consumed by a wireless network interface are presented. The first technique uses a novel dynamic contention window control scheme to improve the performance and energy efficiency of IEEE 802.11 DCF (Distributed Coordinated Function) wireless networks. This al,lows a transmitter to dynamically vary the contention window size in ad-hoc wireless networks to reduce the energy induced by retransmissions caused by packet collisions. The second technique uses a rate adaptation scheme that varies transmission power in order to reduce the energy consumed by retransmissions caused by transmission errors in the radio channel. The third technique involves energy conservation for routing in ad-hoc and multi-hop networks, whkh uses a cross-layer framework and operates in the PHY, MAC and Network layers. With this technique, dynamic power control is used to produce an efficient protocol design for multi-hop wireless networks in the physical layer, and a cross-layer routing algorithm is' used to provide a balance between energy-efficient transmission and a fair distribution of energy consumption across the nodes involved in a route. A cross-layer design for power-efficient wireless communication is then proposed which is based on these energy conservation techniques. This is the first time that the routing protocol, topology control, rate adaptation and dynamic contention window control for the MAC, have been considered to work in an integrated manner. This research also explicitly considers the physical link quality ofcommunication as an indication of the achievable reduction in energy consumption by ~sing the cross-layer framework design for ad-hoc and multi-hop wireless networks. The performance of the cross-layer design is evaluated through theoretical analysis as well}s extensive simulation, and the results show that the power-efficient cross-layer design is both effective and efficient.