This perspective focuses on reaction mechanisms of hydrogen (H 2) evolution with homogeneous and heterogeneous catalysts. First, photocatalytic H 2 evolution systems with homogeneous catalysts are discussed from the viewpoint of how to increase the efficiency of the two-electron process for the H 2 evolution via photoinduced electron-transfer reactions of metal complexes. Two molecules of the one-electron reduced species of [Rh III(Cp*)(bpy)(H 2O)](SO 4) (bpy = 2,2′-bipyridine) and [Ir III(Cp*)(H 2O)(bpm) Ru II(bpy) 2](SO 4) 2 (bpm = 2,2′-bipyrimidine) produced by photoinduced electron-transfer reactions are converted to the two-electron reduced complexes suitable for H 2 generation by disproportionation. The photocatalytic mechanism of H 2 evolution using Pt nanoparticles as a catalyst is also discussed based on the kinetic analysis of the electron-transfer rates from a photogenerated electron donor to Pt nanoparticles, which are comparable to the overall H 2 evolution rates. The electron-transfer rates become faster with increasing proton concentrations with an inverse kinetic isotope effect, when H + is replaced by D +. The size and shape effects of Pt nanoparticles on the rates of hydrogen evolution and the electron-transfer reaction are examined to optimize the catalytic efficiency. Finally, catalytic H 2 evolution systems from H 2 storage molecules are described including shape dependent catalytic activity of Co 3O 4 particles for ammonia borane hydrolysis and a large tunneling effect observed in decomposition of formic acid with [Ir III(Cp*)(H 2O)(bpm)Ru II(bpy) 2](SO 4) 2.