This paper presents an overview of recent research and developments in passive radars at Warsaw University of Technology. The main goal is to show potential of passive radars to provide wide area covert surveillance to protect sensitive infrastructure and strategic objects (e.g. embassies, airports, power plants, railway station, etc.). The study presented in the paper shows that passive radar technologies using different commercial emitters of opportunity, such as FM radio, DAB, DVB-T, GSM, WiFi, WiMax, have obtained maturity stage and they can be successfully used as primary or auxiliary sensor for public security.