This paper describes a novel method of stress determination in structures under dynamic loading conditions. The principle is based on the thermodynamic property of a material in which, under adiabatic conditions, heat is produced or absorbed in direct proportion to the magnitude of the stresses to which it is being subjected. An instrument is described which uses infra-red radiation to measure, remotely, the resulting minute local temperature changes in the material, with oscillating mirrors to provide a raster type scan of the item under test. Spatial resolution down to 1 mm and a temperature discrimination of about 0.002 00 have been achieved, this latter representing a stress change of less than 300 lb/in2 in steel. Results are given which demonstrate the ease with which the equipment can be used to assist in the design of a structure and an indication is given of its potential value in the field of Non-Destructive Testing, for example in the location of fatigue cracks.