Only a small part of the numerous photochemical reactions in organic compounds result in reversible structural changes. The latter are accom­ panied by a change in physical properties, in particular, of electronic spectra. It is tempting to try to use this photochromic effect in various systems for controlling and regulating light fluxes and for data recording. Eventually an independent trend emerged in photochemistry - the study of the photochromism of organic compounds to establish relationships between structure and photochemical behavior, the theoretical analysis of these relationships to predict structures with pre-set photochromic parameters, and, finally, the identification of suitable areas for the application of photochromism. This monograph summarizes the results of recent studies carried out by a number of research institutions in the USSR and the GDR. Devoted to an important aspect of applied photochemistry, this monograph contains a detailed exposition of the chemical photochromes referred to briefly in the earlier editions of "Introduction to the Photochemistry of Organic Compounds" (Khimiya Press, Moscow, 1976) and "Photochemical Proces­ ses in Layers" (Khimiya Press, Moscow, 1978).