Abstract: To measure the temperature of a sub-surface volume of a body, for example, living tissue, in microwaves, at least two probes are placed on or near the surface of the body to pick up thermal noise in the microwave range. The two probes are positioned so that the volumes of the body from which the respective probes receive thermal noise signals overlap to provide a common volume from which both of the probes receive thermal noise signals. The signals received by the probes are processed so as to be amplified and to accentuate the signals emitted by the common volume relative to the signals omitted by the remainder of the volumes from which the probes receive thermal noise signals. In one form of apparatus, the two probes are connected in parallel to a correlator the output of which is connected to an amplifier-receiver. In another form of apparatus the probes are connected to an amplifier-receiver, one being connected through a phase shifter.