Advances in wireless communication have enabled mobility of personal computing devices equipped with sensing and computing capabilities. This has motivated the development of location-based services (LBS) that are implemented on top of existing communication infrastructures to cater for changing user contexts. To enable and support the delivery of LBS, accurate, reliable and real time user location information is needed. This book introduces localization system for tracking the position of mobile node, using received signal strength (RSS) in Wireless Local Area Networks (WLAN).The main challenge in WLAN positioning is the unpredictable nature of the RSS-position relationship. Existing system relies on a set of training samples, collected at a set of test points with known positions in the environment to characterize this relationship.