Industrialization and modernization in recent times have led to a water crisis across the world. Conventional methods of water treatment like physical, chemical and biological methods which comprise of many commonly used techniques like membrane separation, adsorption, chemical treatment etc. have been in use for many decades. However, problems like sludge disposal, high operating costs etc. have led to increased focus on Advanced Oxidation Processes (AOPs) as alternative treatment methods. AOPs basically involve reactions relying on the high oxidation potential of the hydroxyl (OH•) free radical. They have the potential to efficiently treat various toxic, organic pollutants and complete degradation of contaminants (mineralization) of emerging concern. Many different types of homogenous as well as heterogenous AOPs have been studied viz: UV/H2O2, Fenton, Photo-Fenton, Sonolysis, Photocatalysis etc. for treatment of a wide variety of organic pollutants. Different AOPs are suitable for different types of wastewater and hence proper selection of the right technique for a particular type of pollutant is required. The inherent advantages offered by AOPs like elimination of sludge disposal problems, operability under mild conditions, ability to harness sunlight, non selective nature (ability to degrade all organic and microbial contamination) etc. have made it one of the most actively researched areas in recent times for wastewater treatment. Despite the benefits and intense research, commercial applicability of AOPs as a practical technique for treating wastewater on a large scale is still far from satisfactory. Nevertheless, positive results in lab scale and pilot plant studies make them a promising water treatment technique for the future. In the present chapter, an attempt has been made to discuss all aspects of AOPs beginning with the fundamental concepts, classification, underlying mechanism, comparison, commercialization to the latest developments in AOPs.