Wireless LANs are becoming increasingly popular today, particularly those based on IEEE 802.11b standard. We study the problem of determining the location of a mobile device, which is communicating through a WLAN. We exploit the fact that the strength of the signals that a device will receive from different access points will vary with location. We build a database of signal strength information for various locations, and use this information to determine which location a given test data comes from. The problem is complicated because RF signals are affected by the noise, interference, multi-path effect, and random movement in the environment. We find that in spite of this randomness, the signal information is sufficient to detect the position of mobile device with certain error margin. © 2003 IEEE.