Two mechanisms of hydrogen transfer are often observed in photochemical reactions. In a one-step procedure, the electron and the proton are simultaneously transferred (concerted process). In a two-step procedure, first an electron is transferred and the proton follows. Such steps are observed in photochemically induced radical reactions with α,β-unsaturated carbonyl and carboxyl compounds in which a radical species is added at either the α- or the β-position. Both mechanistic steps are also observed in photochemical reactions of imides. In Norrish­–Yang-type reactions, especially with aromatic carbonyl compounds, the spin multiplicity has an influence on the resulting cycloadditions. Such reactions are interesting tools for the synthesis of natural products and for diversity-oriented synthesis. Photochemically induced hydrogen transfer in photoredox catalytic reactions is discussed in connection with proton-coupled electron transfer.