Biological processes are usually most efficient for degrading pollutants occurring in wastewater. Refractory and toxic compounds contained limit their applicability. In such cases combinations with chemical oxidation processes may improve the overall efficiency and efficacy. Most suitable oxidation processes for combination with biological treatment are wet air oxidation, ozonation, hydrogen peroxide treatment and other advanced oxidation processes. Most effective are OH-radicals produced in all these oxidation processes. Chemical oxidation produces intermediates with usually improved biodegradability. Process combinations may be serial or with recycling between chemical oxidation and biological treatment. Design criteria, control of combined processes and recent applications are reviewed.