The photochemical aspects of flavins have been reviewed with reference to the analysis of flavins and photoproducts in degraded solutions, spectral and photophysical properties, photodegradation reactions, photosensitization reactions, and photostability of riboflavin in pharmaceutical preparations. The photochemistry of flavins in aqueous solution involves two major photodegradation reactions, viz intramolecular photoreduction and photoaddition, occurring separately or concomitantly in the presence of phosphate buffer, leading to the formation of lumichrome and cyclodehydroriboflavin, respectively. The photoreduction is catalyzed by H₂PO₄- ions and the photoaddition by HPO₄²⁻ ions. Intermolecular photoreduction also takes place in a large number of flavin-substrate interactions involving the flavin triplet state. Knowledge of the spectral and photophysical properties, and photochemical behaviour of flavins is necessary to understand the nature of flavin–protein interactions, flavin photosensitization reactions, and their role in biological processes. The study of the photochemical behaviour of riboflavin as a function of pH is a prerequisite to achieve optimum stability of the vitamin in pharmaceutical preparations. Technological processes are being developed to utilize riboflavin as a photosensitizer in the pathogenic inactivation of blood components. Riboflavin-sensitized photooxidation of amino acids in protein may have important biological implications to control photoinduced damage in biological systems. The application of femtosecond-resolved fluorescence spectroscopy and transient absorption techniques may provide useful information on the dynamics of flavoproteins.