This chapter focuses on photocatalytic mechanisms of hydrogen (H2) evolution to clarify how photons are converted to two electrons that are required for H2 production from two protons. A two-electron-reduced metal complex is produced via disproportionation of the one-electron-reduced species of a metal complex produced via photoinduced electron transfer, leading to H2 evolution. A one-photon two-electron process is made possible in photocatalytic H2 evolution by combination of thermal and photoinduced electron transfer. Photoexcitation of 9-mesityl-10-methylacridinium ion (Acr+–Mes) with NADH that is a hydride (two-electrons and a proton) donor resulted in the reduction of two equivalents of Acr+–Mes to produce two equivalents of Acr•-Mes that reduce protons to produce H2 in the presence of an H2 evolution catalyst. Acr+-Mes can also be applied to photocatalytic generation of H2, accompanied by dehydrogenative oxygenation of an alkene and selective C(sp2)-H amination of arenes. A one-photon two-electron process is also made possible by a bimolecular reaction of the excited state of a metal-hydride complex with the ground state complex to produce H2.