The state of the art in electrochromic materials is briefly reviewed. These materials have potential for architectural and automotive applications for they provide control of radiant solar energy transmission. Optical properties of these materials can be reversibly changed by electrochemical oxidation and reduction. These optically switchable materials can provide control of solar heat load on air conditioning systems, reduce glare for rear view mirrors, and enhance privacy in an automobile. The main features of the electrochromic characteristics of the best-known groups of materials are described. The properties of some of the well-known materials such as WO3, Ni (OH)2, viologens, polyaniline and Prussian Blue are discussed with particular reference to the mechanism of electrochromic reactions.