Heterogeneous photocatalysis is a novel technique for water purification. Publications on photocatalysis span a relatively recent period of not more than 25 years. This is a technique that, according to our extensive experience on the development of laboratory scale and pilot plant units, has great promise to eliminate water and air pollutants. Photocatalysis offers much more than competitive techniques where pollutants are transferred from phases; photocatalysis can achieve complete mineralization of pollutants leaving non-toxic species such as CO2 and H2O and can be exploited at close to room temperature and ambient pressure.