This chapter describes the architecture of a typical WLAN. It discusses the difference between a WLAN and a cellular network. The chapter identifies he key factors influencing WLAN performance. It also describes Institute of Electrical and Electronic Engineers 802.11 physical layer characteristics. The chapter compares 802.11 distributed coordination function and point coordination function access mechanisms. It provides an introduction to various key concepts of WLANs that are necessary for the design, deployment, and performance evaluation of such systems. There are two commonly used network architectures for the design of WLANs: ad hoc network and infrastructure network. The performance of a typical WLAN depends on various factors, including radio propagation characteristics and environments, medium access control protocols, and traffic type and their distribution. Due to the broadcast nature of radio signals, the location of the transmitting and the receiving antenna is important in determining the performance of WLANs.