Primary photoreactions of riboflavin (Rf) are considered. The goal is to present data on the transients formed under photoexcitation of Rf: excited singlet and triplet states, triplet reactions with the formation of Rf neutral free radical, and a radical-anion. The absorption spectra of transients are presented, as are the rate constants of their decay. Special attention is devoted to Rf in water (aerobic and anaerobic reactions).