Abstract: Microwave thermography apparatus comprises a signal-receiving aerial (2) which is sensitive to microwave radiation emanating from a body having a mean body temperature. A reference load (1) capable of being maintained at a chosen temperature (T.sub.O) and of emitting a thermal noise signal at microwave frequency corresponding to temperature T.sub.O, is connected to a receiver (3) the input impedance of which is matched to the impedance of reference load (1) by way of a 3-port non-reciprocal circulator (5). Aerial (2) is connected via a single-throw on/off microwave switch (6) to the input of receiver (3) through the other port of circulator (5). The connections to circulator (5), taken in circulation order, are reference load (1), aerial (2), receiver (3). Aerial (2) is preferentially designed to incorporate a tubular metallic waveguide (10) of circular cross-section dimensioned to support TE.sub.