Abstract : Wireless Local Area Network abbreviated as (WLAN) is the new generation of Local Area Networks (LANs) that is to satisfy the mobility and flexibility needed by recent users (mostly laptop users). This thesis presents the theory of IEEE 802.11 standards of WLAN, and discusses various WLAN performance parameters and their configuration. Simulation projects for different WLAN topologies have been built using OPNET network simulator. These projects are mainly Basic Service Set (BSS) topology, and Extended Service Set (ESS) topology simulated under different parameters values (access mechanism, data rate, fragmentation, RTS/CTS, number of nodes, packet reception power, load condition, and mobility). Simulation results show what access mechanism is better under what load condition, also how to choose best fragmentation threshold and other access-mechanism specific parameters according to the network conditions. The results also show the effect of packet reception power on the BSS cell size.