Section headings. General introduction. Photophysical, photochemical and photokinetic properties of photochromic systems. Photochromism based on "E-Z" isomerization of double bonds. Photochromism based on pericyclic reactions: electrocyclization reactions. Photochromism based on pericyclic reactions: cycloaddition reactions. Photochromism based on tautomerism (hydrogen transfer). Photochromism based of dissociation processes. Photochromism in biological systems. Environmental effects on organic photochromic systems. The use of silver salts for photochromic glasses. Applications. New developments highly promising for applications.