Abstract: Method of detecting and tracking an unmanned aerial vehicle, the method comprising, at a detector unit (300a) comprising a first microphone and a second microphone: monitoring for a sound associated with the presence of the unmanned aerial vehicle (505) in the vicinity of the detector unit; in response to the monitoring indicating the presence of the unmanned aerial vehicle, determining, at the detector unit, a phase delay between the sound as received at the first microphone and the sound as received at the second microphone; on the basis of the determined phase delay and a known separation of the first microphone and the second microphone, determining, at the detector unit, an azimuth angle (507a) to the unmanned aerial vehicle from the detector unit; and transmitting, to a computing node (501), the determined azimuth angle for use in determining a location of the unmanned aerial vehicle.