To understand the details of photocatalytic reactions, it is essential to reveal the kinetic processes of photogenerated charged carriers in photocatalysts. This chapter gives a compact review of primary processes in photocatalysis. First, the photocatalytic processes is outlined along with the redox energies of molecular oxygen and water in addition to the band energy of TiO2. The dynamics of photoinduced free carriers, i.e., conduction band electrons and valence band holes to be a trap at the surface, and the decay of the trapped carriers with O2 and alcohols are described. Then, kinetic processes for the formation of ˙O2−, H2O2, and ˙OH at the TiO2 surface are summarized. Finally, the behaviour of photoinduced electrons and holes is presented for visible light responsive TiO2 such as Cu(ii) deposited TiO2, Fe(iii) deposited Ru-doped TiO2, Pt-complex sensitized TiO2, and gold-nanoparticle sensitized TiO2.