Many studies have shown the important role of defect chemistry in TiO2 photocatalysis. Oxygen defects are the defects intrinsically existing in TiO2. It has been reported that oxygen defects on TiO2 surface could promote O2 activation for organic oxidations by mediating electron transfer. Oxygen defects in the bulk mainly act as recombinationcenters because they are far from reactants in distance.However, it has been also reported that the electrons denoted from bulk oxygen defects could promote chemical adsorption of O2 on TiO2 surfaces and is thus good for photocatalysis. In addition to TiO2, oxygen defects are important for photocatalysis of other oxides such asZnO and WO3. Furthermore, a synergism between oxygen defects on surfaces and in the bulk was also proposed in a benzene photocatalytic oxidation over TiO2. Ti3+ ions, resulting fromelectron localization, are usually regarded as self-dopants of TiO2 because they can lead to intermediated states within TiO2 gap. The role of Ti3+ defects in charge carrier kinetics has been investigated.For example, the increase of the H2 photocatalytic generation over TiO2 was attributed to an increase in electron conductivity caused by Ti3+ states.Ti3+ states in principle belong to polaronic states and mainly reside in the bulk because the surface ones are easily subject tounavoidable oxidation.