The range of infrared (IR) detectors at present available is described and a comparison is made between their performances with reference to particular applications. Due to the increasing availability and use of IR sensors the problems involved in data interpretation are becoming more widely acknowledged. Hence the major part of this paper is devoted to a detailed survey of these problems and offers a guide to those wishing to use IR data either quantitatively or qualitatively. Throughout, special reference is made to airborne IR surveys and their application to building heat loss.