Smart phones have become an important part of our daily lives due to their capabilities of accessing the web using WiFi and mobile data networks. These WiFi equipment are constantly sending out packets referred as probe requests, which can be tracked using wireless sniffers. In this thesis, first we investigate capturing of WiFi probe request packets using the help of WiFi Pineapple devices, and analyze how we can use signal strength information of probe request data for indoor occupancy monitoring. Applications of such occupancy monitoring into building surveillance and building energy management are also discussed.