Waste water effluents coming from textile and other dyestuff industries create increasing environmental hazard. These effluents will pose serious effects to the aquatic ecosystem, if they haven't properly treated before discharging into water stream. Hence, it is necessary to evolve immediate steps to control water pollution and safeguard our environment and mankind. Among the various water and wastewater treatment methods, advanced oxidation processes such as Heterogeneous Photocatalysis, Fenton, Photo-Fenton, Sonophotocatalysis and Ozonation are the most promising techniques for the decolorisation and degradation of dyes completely into mineralized products, CO2 and H2O.In this book, general introduction, water pollution, terminologies such as pollution and pollutant, types of pollution, types of pollutants, water pollution, water pollutants, sources of water pollution, types of water pollutants, methods of removal of water pollutants, advanced oxidation processes, various methods of generating hydroxyl radicals and their mechanism of generation are presented.