Abstract: Temperature, or temperature noise, is measured by employing an ultrasonic beam which strikes at least one pair of points (10A, 10B) which are a known distance apart and the time interval between the two points is resolved and related to the distance to give a measure of temperature of the medium between the two points. The point pairs may be naturally occurring, such as the diametrically opposed edges of a fuel element channel in a sodium-cooled nuclear reactor, or specifically provided. The beam may be at glancing incidence on a number of point pairs in line and, in this situation, the beam is wider in the direction transverse to its path than it is along its path.