Photo-induced redox chemical reactions occurring on irradiated semiconductor surfaces have been utilized for the purification of water contaminated with various inorganic and organic chemicals. Here, we focus on TiO2 as the most popular photocatalyst and briefly describe its characteristics and applications mainly in relation with the photochemical redox conversion of aquatic pollutants. The photoexcitation of TiO2 induces electron-hole pair formation and subsequent charge separation/migration/ transfer leads to the production of highly reactive oxygen species (ROS) such as OH radical and superoxide on the surface of TiO2. Aquatic organic pollutants subsequently react with ROS, holes, or electrons, and they undergo a series of redox chemical reactions, eventually leading to mineralization. The photo-induced ROS generation on TiO2 is exploitable for bacterial/viral inactivation as well, while TiO2 particles at the nano-and microscale possibly induce adverse biological effects in the absence of light. Photo-induced redox reactions on TiO2 can also transform a variety of inorganic pollutants such as oxyanions (arsenite, chromate, bromate, etc.), ammonia, and metal ions. On the other hand, the photocatalytic degradation mechanism can be actively controlled by modifying the surface of TiO2 to change the products. For example, the photocatalytic degradation of phenolic compounds can be accompanied by the simultaneous production of hydrogen when the surface of TiO2 is modified with both platinum and fluoride. Finally, the photocatalytic activity of TiO2 is highly dependent on the kind of substrates and the activity assessed with a specific test substrate is difficult to generalize. Therefore, the photocatalytic activities of TiO2 should be assessed using multiple substrates to obtain balanced information.