A system (100; 300; 904) for standoff detection of human carried explosives within an area under surveillance, comprising: an active radar system, the radar system having a radar transmitter, a radar receiver, at least one radar antenna, and a field of regard (118; 914), wherein the at least one radar transmitter is arranged to transmit a narrow beam of polarised radiation towards the field of regard (118; 914); a radar tracking control system operatively coupled to the active radar system, wherein the field of regard (118; 914) of the active radar system is adapted to be targeted within the area under surveillance, the area of surveillance being greater than the radar field of regard (118; 914); a processor (102; 302) operatively coupled to the radar system and the radar tracking control system; wherein the processor (102; 302) is adapted to provide a radar tracking instruction for targeting the radar system, to provide detection instructions for detecting human carried explosive devices and to detect human carried explosives by detecting a differential between co-polarized and cross-polarized reflections from a target; a video display terminal coupled to the processor (102; 302); and an imaging system coupled to the processor (102; 302), the imaging system having a field of regard (118; 914) adapted to spatially and temporally overlap the radar system field of regard, wherein the imaging system has at least one video output coupled to the processor, wherein the processor (102; 302) is adapted to detect and track a moving human target from the video output signal, and wherein the system for standoff detection is adapted to control the imaging system to maintain a track of the moving human target and wherein track headings of the moving human target detected in the video output signal are used to control the radar beam position and to trigger the radar, and wherein the processor (102; 302) is adapted to compute a running sum of the differential between co-polarized and cross-polarized reflections of the reflected radar signal over multiple observations of the moving human target and wherein the detection of human carried explosives is based on the running sum by comparing the running sum to reference data, and wherein the processor is adapted to continue computing the running sum and comparing the running sum to the reference data until a threat or non-threat determination of the moving human target can be made. an active radar system, the radar system having a radar transmitter, a radar receiver, at least one radar antenna, and a field of regard (118; 914), wherein the at least one radar transmitter is arranged to transmit a narrow beam of polarised radiation towards the field of regard (118; 914); a radar tracking control system operatively coupled to the active radar system, wherein the field of regard (118; 914) of the active radar system is adapted to be targeted within the area under surveillance, the area of surveillance being greater than the radar field of regard (118; 914); a processor (102; 302) operatively coupled to the radar system and the radar tracking control system; wherein the processor (102; 302) is adapted to provide a radar tracking instruction for targeting the radar system, to provide detection instructions for detecting human carried explosive devices and to detect human carried explosives by detecting a differential between co-polarized and cross-polarized reflections from a target; a video display terminal coupled to the processor (102; 302); and an imaging system coupled to the processor (102; 302), the imaging system having a field of regard (118; 914) adapted to spatially and temporally overlap the radar system field of regard, wherein the imaging system has at least one video output coupled to the processor, wherein the processor (102; 302) is adapted to detect and track a moving human target from the video output signal, and wherein the system for standoff detection is adapted to control the imaging system to maintain a track of the moving human target and wherein track headings of the moving human target detected in the video output signal are used to control the radar beam position and to trigger the radar, and wherein the processor (102; 302) is adapted to compute a running sum of the differential between co-polarized and cross-polarized reflections of the reflected radar signal over multiple observations of the moving human target and wherein the detection of human carried explosives is based on the running sum by comparing the running sum to reference data, and wherein the processor is adapted to continue computing the running sum and comparing the running sum to the reference data until a threat or non-threat determination of the moving human target can be made.