Photochromic materials, which react reversibly to light, may be distinguished from the familiar photographic materials, which, because of the chemical development of the image, are essentially irreversible. Many organic photochromics, and a few inorganic ones, are known. Their general properties are described, with particular emphasis on those of a system of photochromic glasses. The behaviour of these glasses is that of silver halide crystals deliberately formed in the glass during its manufacture. They show relatively large variation 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, will be described, and some applications for them will be suggested.