Advanced oxidation processes (AOPs), in a broad sense, are a set of chemical treatment procedures designed to remove organic (and sometimes inorganic) materials in water and wastewater by oxidation through reactions with hydroxyl radicals (·OH). In practice within wastewater treatment, this term usually refers more specifically to a subset of such chemical processes that employ ozone (O₃), hydrogen peroxide (H₂O₂) and UV light or a combination of the few processes. Common AOP configurations often include Fenton and photo-Fenton systems, in addition to ozone/UV, TiO₂/UV photocatalysis, and Electro-Fenton systems.

AOPs rely on in-situ production of highly reactive hydroxyl radicals (·OH) or other oxidative species for oxidation of contaminant. These reactive species can be applied in water and can oxidize virtually any compound present in the water matrix, often at a diffusion-controlled reaction speed. Consequently, ·OH reacts unselectively once formed and contaminants will be quickly and efficiently fragmented and converted into small inorganic molecules. Hydroxyl radicals are produced with the help of one or more primary oxidants (e.g. ozone, hydrogen peroxide, oxygen) and/or energy sources (e.g. ultraviolet light) or catalysts (e.g. titanium dioxide). Precise, pre-programmed dosages, sequences and combinations of these reagents are applied in order to obtain a maximum •OH yield. Researchers are also exploring doped metal-oxide catalysts and carbon-based materials to boost radical yields and broaden pH operating windows. In general, when applied in properly tuned conditions, AOPs can reduce the concentration of contaminants from several-hundreds ppm to less than 5 ppb and therefore significantly bring COD and TOC down, which earned it the credit of being one of the most effective "water treatment processes of the 21st century". Current research also focuses on reducing energy demand and minimizing byproducts produced during the process—such as bromate—to improve the viability of AOP implementation in different industries.

The AOP procedure is particularly useful for cleaning biologically toxic or non-degradable materials such as aromatics, pesticides, petroleum constituents, and volatile organic compounds in wastewater. Additionally, AOPs can be used to treat effluent of secondary treated wastewater which is then called tertiary treatment. The contaminant materials are largely converted into stable inorganic compounds such as water, carbon dioxide and salts, i.e. they undergo mineralization. A goal of the wastewater purification by means of AOP procedures is the reduction of the chemical contaminants and the toxicity to such an extent that the cleaned wastewater may be reintroduced into receiving streams or, at least, into a conventional sewage treatment.

Although oxidation processes involving ·OH have been in use since late 19th century (such as Fenton's reagent, which was used as an analytical reagent at that time), the utilization of such oxidative species in water treatment did not receive adequate attention until Glaze et al. suggested the possible generation of ·OH "in sufficient quantity to affect water purification" and defined the term "Advanced Oxidation Processes" for the first time in 1987. AOPs still have not been put into commercial use on a large scale (especially in developing countries) even up to today mostly because of relatively high associated costs. Nevertheless, its high oxidative capability and efficiency make AOPs a popular technique in tertiary treatment in which the most recalcitrant organic and inorganic contaminants are to be eliminated. The increasing interest in water reuse and more stringent regulations regarding water pollution are currently accelerating the implementation of AOPs at full-scale. There are roughly 500 commercialized AOP installations around the world at present, mostly in Europe and the United States. Other countries like China are showing increasing interests in AOPs.^{[citation needed]}

The reaction, using H₂O₂ for the formation of ·OH, is carried out in an acidic medium (2.5-4.5 pH) and a low temperature (30 °C - 50 °C), in a safe and efficient way, using optimized catalyst and hydrogen peroxide formulations.

Generally speaking, chemistry in AOPs could be essentially divided into three parts:

The mechanism of ·OH production (Part 1) highly depends on the sort of AOP technique that is used. For example, ozonation, UV/H₂O_2, photocatalytic oxidation and Fenton's oxidation rely on different mechanisms of ·OH generation:

Fe²⁺ + H₂O₂ → Fe³⁺+ HO· + OH⁻ (initiation of Fenton's reagent)