An object localising system comprises sensor devices at different sites, each sensor device being capable of detecting a signal from an object, and control means for repeatedly responding to the outputs of the sensor devices by selecting a sub-set of the devices and determining the amount by which the times at which the devices of the sub-set receive the signal are delayed with respect to each other to enable calculation of the current location of the object. Each sensor device can be switched between a master mode, in which the device is operable to transmit events derived from a signal from an object, and a slave mode, in which the device is responsive to such events from another sensor device for processing its own signal in order to determine the time delay between receipt of the signals by the sensors of the respective devices.