||Wireless Local Area Networks (WLANs) has amazingly evolved and attracted much attention. Due to the simplicity and low cost, ease of deployment, robustness and mobility support, IEEE 802.11 WLANs are widely used and become the dominating WLAN technology. In IEEE 802.11 WLANs, there are two types of access modes that perform very differently under different channel or traffic conditions: the basic access mode and the RTS/CTS access mode. We compare the throughput and delay performance of these two access modes via an analytical model, and reveal that different modes should be chosen to obtain better throughput and delay performance under different conditions. Consequently, We propose an adaptive RTS/CTS mechanism that adjusts the RTS threshold dynamically to achieve optimal performance. In order to fulfill the ever-increasing demand for the Quality of Service (QoS) support in WLANs, a new standard IEEE 802.11e is specified. The fundamental and mandatory mechanism, named Enhanced Distributed Channel Access (EDCA), provides relative QoS for differentiated traffic. A comprehensive performance analysis and study of the EDCA is reported in this thesis. We present our development of an analytical model, in which most new features of the EDCA are taken into account. Based on the model, we analyze the throughput performance of differentiated service traffic, and propose a recursive method capable of calculating the mean access delay. Service differentiation functionality and effectiveness of the EDCA are also investigated. Since the EDCA only support non-guaranteed relative QoS, we propose a, Measurement-assisted Model-based Call Admission Control (MM-CAC) scheme for the EDCA to support better performance and more stringent QoS. This MMCAC scheme is based on the non-saturation modelling and analysis work. The non-saturation model we developed is for the legacy IEEE 802.11 Distributed Coordination Function (DCF), however, by introducing a concept of equivalent number of competing entities, it is employed for the CAC design for the EDCA effectively, and avoid the complicated computation in the exact EDCA modelling and analysis to ensure the scheme feasible and efficient to be implemented in a, real-time manner. Finally, we present our development of the formulation for multi-rate WLAN MAC protocol as an utility function optimization problem. Based on this formulation, we analyze the efficiency and fairness performances of different MAC protocols for multi-rate WLANs. To improve system efficiency and fairness of medium sharing, we propose two schemes, namely Limited Transmission Time (LTT) scheme and Multiple Packet Delivery (MPD) scheme, to provide proportional fairness in MAC layer for multi-rate WLANs. We also propose a generic algorithm, called Optimal Contention Window Adaptation (OCWA) algorithm, which by choosing different utility functions, can achieve different preferences between efficiency and fairness for multi-rate WLANs through adjusting the Contention Window (CW) size of each STA in a distributed way. Both QoS provisioning and cross-layer optimization are also discussed in details within this framework.