The reactive intermediate in Advanced Oxidation Processes (AOPs) is the •OH radical. It may be generated by various approaches such as the Fenton reaction (Fe2 + /H2O2), photo-Fenton reaction (Fe3 + /H2O2/hν), UV/H2O2, peroxone reaction (O3/H2O2), O3/UV, O3/activated carbon, O3/dissolved organic carbon (DOC) of water matrix, ionizing radiation, vacuum UV, and ultrasound. The underlying reactions and •OH formation efficiencies are discussed. The key reactions of •OH radicals also addressed in this review.