Wet air oxidation is one of the available technologies for the treatment of aqueous wastewaters. In wet air oxidation aqueous waste is oxidized in the liquid phase at high temperatures (400-573 K) and pressures (0.5-20 MPa) in the presence of an oxygen-containing gas (usually air). The advantages of the process include low operating costs and minimal air pollution discharges, while the main limitations are the high capital costs and safety implications associated with a system operating at such severe operating conditions. As a consequence, significant development in wet air oxidation technology has concentrated on methods of reducing the prohibitive capital costs. In the design of the process a balance must therefore be made between the enhancement of overall reaction rates with temperature and pressure against their effect on capital cost and operational difficulties such as corrosion and scaling of equipment. In this paper the wet air oxidation process is introduced and a number of commercial and emerging technologies presented. These technologies employ a variety of methods to ameliorate the limitations of the technology whilst maintaining acceptable overall reaction rates. These include methods to improve mass transfer as well as the use of both homogeneous and heterogeneous catalysts to enhance reaction rate.