Cytochromes P450 (P450s) catalyze monooxygenation of a wide range of less reactive organic molecules under mild conditions. By contrast with the general reductive oxygen activation pathway of P450s, an H2O2-shunt pathway does not require any supply of electrons and protons for the generation of a highly reactive intermediate (compound I). Because the low cost of H2O2 allows us to use it in industrial-scale synthesis, the H2O2-shunt pathway is an attractive process for monooxygenation reactions. This review focuses on the P450-catalyzed monooxygenation of organic molecules using H2O2 as the oxidant.