Photochromic materials are those which react reversibly to light. Photochromism is therefore distinguished from the familiar photographic processes, which, because of the chemical development of the image, are essentially irreversible. Many organic materials, and a few inorganic ones, show this phenomenon. Their general properties are described, with particular emphasis on those of a system of photochromic glasses. The behaviour of these glasses results from reaction of light with silver halide crystals deliberately formed in the glass during its manufacture. The mechanism of the photolytic reactions is postulated, and compared with the theory of the photographic process for silver halides. Reversibility of photochromism in these glasses is explained by prevention of diffusion of the products of photolysis from the original crystal site within the glass structure, and their subsequent recombination when the activating light is removed. These glasses show large ranges in all their photochromic properties resulting from ranges in composition, and in size and number of the included crystals. The behaviour of typical glasses, in both sunlight and under artificial sources, is reported in this paper, and some applications for these glasses are suggested.