A wireless local area network (WLAN) is a wireless computer network that associates two or more devices by a wireless distribution method within a limited area (for example a home, school, computer laboratory, or office building). This gives users the ability to move around within a local coverage area and still be connected to the network, and can provide a connection to the wider Internet. Most modern WLANs are based on IEEE 802.11 standards, marketed under the Wi-Fi brand name. Wireless LANs have become popular in the home due to ease of installation and use, and in commercial complexes offering wireless access to their customers. The basic structure of all networks consists of a main computer or server, along with connected machines known as clients. The server typically has two Ethernet network interface cards (NICs) installed and software that can support the network. Microsoft Windows operating systems have featured built-in networking capability since Windows 98 Special Edition (SE), but third party networking software is also available. In the case of a simple home wireless LAN, a desktop might be the server while a laptop could be the client. At the office, a wireless LAN provides instant connectivity to mobile personnel. It also avoids the costly expense of running Ethernet cable throughout a building, providing easy, effortless desktop connectivity between clients. Because there are no wires running to the clients, one of the main advantages of a wireless LAN is easy installation. Wireless LANs also provide more flexibility than wired LANs and are less expensive. This book entitled Advanced Wireless LAN offers some of the cutting-edge development status of wireless LAN, covering the topics on physical layer, MAC layer, QoS and systems. It provides an opportunity for both practitioners and researchers to discover the problems that arise in the rapidly developed technologies in wireless LAN.