Abstract: A system for detecting and tracking movement within a building has a plurality of portable microwave detector units for deploying outside the walls of the building. Each detector unit has a microwave module for transmitting microwaves through the building walls into a monitored area and for detecting radiation reflected back to the unit from the monitored area, and an output device for transmitting an alarm signal from the unit if a shift is detected in the reflected radiation frequency, indicating movement within the area. A receiver unit receives alarm signals from the detector units and produces an output signal for each detector unit currently detecting movement to provide the user with information on the location of any movement within the building.