Abstract: A tyre running hot due to under-inflation or other causes radiates heat. This is detected by sensors (4,27) positioned within the wheel arches. A comparison is made between each sensor output and a norm, such as ambient temperature from air temperature detectors (5) or the mean of all the sensor outputs, and significant departure from that norm triggers an alarm (22). The sensors may each be a detector (27) at the focus of a parabolic reflecting shell (28) shielded by a heat-transparent cover (30), and they will generally be positioned to face the inner side walls of the tyres. Direct wiring to the alarm unit is possible, but with a trailer (2) the signals may be locally encoded, fed by existing wiring (13) to the tractor (1) and there be decoded.