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IEEE 802.11 has been successfully used as the last-mile technology in the present-day pervasive computing environments. With the broadband capability in IEEE 802.11 networks, it is a natural demand that appropriate QoS strategies are devised in order to better support bandwidth/delay constrained video/voice applications. Depending on the actual applications and types of networks, the issues of QoS can be quite different. However, for all IEEE 802.11 wireless networks, a fundamental issue is interference. With Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), interference may occur when two nodes within the carrier sensing range of each other attempt to transmit data packets through the wireless channel. Combined with factors such as network topology, traffic load, and node mobility, this type of interference makes QoS support in IEEE 802.11 based wireless networks, especially multi-hop ad hoc networks, a challenging issue. In this dissertation, we proposed an admission control model and an interference model in IEEE 802.11 wireless networks. The admission control model gives a general way of enforcing flow acceptance/rejection based on the bandwidth availability at a node/link. Based on the admission control model, we proposed schemes to provide QoS in IEEE 802.11e wireless LANs and end-to-end QoS guarantee in heterogeneous wired/wireless networks with support for roaming among different wireless LANs. The interference model incorporates many important factors such as network topology, link rate, traffic load, and node mobility, and quantifies the node/link interference through two metrics: general interference metric and intra-flow interference metric. By combining the interference metric and the admission control model, we proposed schemes to provide reliable QoS in multi-rate multi-hop ad hoc networks with the help of route reliability information. In addition, the interference model has been applied to other protocols such as routing and topology control to achieve higher network capacity and can be further extended to multi-channel wireless mesh networks (WMNs). Extensive network simulations have been conducted to evaluate the proposed strategies. Results show that the proposed interference aware QoS strategies can sufficiently ensure the bandwidth/delay requirement of flows and thus can support multimedia applications in IEEE 802.11 based wireless networks in a better manner.