Titanium dioxide (TiO₂) powders strongly support photocatalytic oxidation on their surfaces. This oxidation, produced by irradiation with blue visible light and/or near ultraviolet light, effectively decomposes various substances. Various useful applications of TiO₂ photocatalysis have been reported recently in medical and dental fields, and also in environmental fields such as water purification. The bactericidal effect depends on the concentrations of bacterial cells and TiO₂ powder. When the bacterial cell density is low (less than 10⁵ colony-forming units per ml), the cells are killed within a few minutes. When the cell density is high (more than 10⁷ colony-forming units per ml), the cells generate co-aggregation with the TiO₂ particles and the bactericidal action takes much longer. When the concentration of TiO₂ is very high (more than 10 mg ml^-1), the bactericidal effect is not observed. Oxygen is required for the bactericidal effect and scavengers for activated oxygen such as superoxide dismutase and catalase therefore inhibit this effect. The bactericidal action may be attributed to the oxidation of the bacterial membrane with activated oxygen, which is produced from water and oxygen by TiO₂ photocatalysis. In this review the mechanism by which various microorganisms are killed by TiO₂ photocatalysis is discussed and several applications of TiO₂ photocatalysis are considered.