Abstract: The temperature within a body is measured by covering an area of the body surface with a layer of thermal insulation, measuring the temperature with two sensors located respectively at the insulation/body surface interface and at the opposite side of the insulating layer to indicate any temperature gradient along a path from within the body through the sensors, and applying heat over the insulating layer/sensor sandwich to annul any such gradient. This effectively exteriorises the desired deep body temperature, which is then indicated by the sensors. It is important that the insulation extend laterally beyond the sensors to guard against peripheral temperature gradients affecting the sensor region and to take account of any inherent thermal irregularities in the body. This last point is particularly relevant to clinical use on human subjects with irregularities caused by vascular channels. Similar considerations lead to use of a thin film or similar uniform heating element over the relevant area.