A radar system adapted to be installed on a vehicle (1) is operable to detect objects in different fields of view of the system. The system has transceiver means (2, 3; 44, 46) for transmitting a radar signal and receiving the signal after the latter has been reflected from an object to be detected. A reflected signal is sampled by sampling means (12; 66) during a succession of sampling periods each of which is delayed with respect to a corresponding portion of the transmitted signal. The gating of the reflected signal effectively generates range shells, each of which corresponds to the time taken for a transmitted signal to reach an object at the shell, and to be reflected back to the system and is hence related to the delay between the transmission of the signal and the corresponding sampling period. The range shells define the field of view of the system, which can be changed by altering said delay. Such alterations are carried out under the control of control means which is connected to sensor means for detecting a property of the operation of the vehicle, determining a required position of the field of view, and setting the field of view of the system accordingly. The range shell positions, and hence fields of view, can also be changed by selecting alternative transmitters and/or receivers of the transceiver means. There is also shown a motor road vehicle which has a radar system comprising a pair of transmitter antennas at two alternate corner regions of the vehicle and a pair of cooperating receiver antennas of the other two corner regions. Certain embodiments of the system also have the facility to generate range shells which are swept over an area around the vehicle or which track detected objects.