Due to limited availability of GPS-like signals indoors, and prevailing deployment of WLAN infrastructure in these environments, many proposed state-of-the-art indoor positioning techniques operate using a collection of WLAN signal measurements, called wireless fingerprints or just fingerprints that quite uniquely relate to user locations. As WLAN infrastructure was not historically designed for localization, the research community addressed several challenges to achieve robust operation of indoor positioning systems. While there are still other problems that hinder broad deployment of indoor navigators, an accumulated critical mass of scientific knowledge in this area is expected to drastically change indoor location-awareness, similar to the GPS revolution for outdoor navigation. This paper reviews main concepts of WLAN localization for a short introduction to this emerging transformative technology.