Current water-treatment technologies require considerable energy consumption. Thus, closely linked to the problem of water shortage is the impending energy crisis. Therefore, intensive research is being aimed at developing water purification processes that are based upon using renewable energy, such as solar energy, rather than energy generated using fossil fuels. There has been an accumulation of reports on the development of photocatalysts, which enable water purification using solar energy as the only driving force. Such photocatalysts, based upon oxide semiconductors, permit the conversion of solar energy into the chemical energy that is required for the oxidation of toxic organic compounds in water. The most promising photocatalyst is titanium dioxide, TiO2, and its solid solutions. The research on TiO2 photocatalysis is multidisciplinary, and progress in this area requires the application of concepts of catalysis and photocatalysis as well as concepts of solid-state chemistry.