Photocatalysis is based on the double aptitude of the photocatalyst (essentially titania) to simultaneously adsorb both reactants and to absorb efficient photons. The basic fundamental principles are described as well as the influence of the main parameters governing the kinetics (mass of catalyst, wavelength, initial concentration, temperature and radiant flux). Besides the selective mild oxidation of organics performed in gas or liquid organic phase, W-irradiated titania becomes a total oxidation catalyst once in water because of the photogeneration of OH. radicals by neutralization of OH-surface groups by positive photo-holes. A large variety of organics could be totally degraded and mineralized into CO2 and harmless inorganic anions, Any attempt of improving titania＇s photoactivity by noble metal deposition or ion-doping was detrimental. In parallel, heavy toxic metal ions (Hg2+, Ag+, noble metals) can be removed from water by photodeposition on titania. Several water-detoxification photocatalytic devices have already been commercialized. Solar platforms are working with large-scale pilot photoreactors, in which are degraded pollutants with quantum yields comparable to those determined in the laboratory with artificial light.