Anaerobic-aerobic systems have been remarkably employed in industrial and municipal wastewater treatment for many years. While previously most treatment of wastewaters have been carried out in conventional anaerobic-aerobic treatment plants, in recent years, high rate anaerobic-aerobic bioreactors have been increasingly employed for wastewaters with high chemical oxygen demand (COD). This paper provides a review of the various types of high rate anaerobic-aerobic water treatment techniques currently available including high rate bioreactors and integrated anaerobic-aerobic bioreactors. The integrated bioreactors are classified into four types, which are (i) integrated bioreactors with physical separation of anaerobic-aerobic zone, (ii) integrated bioreactors without physical separation of anaerobic-aerobic zone, (iii) anaerobic-aerobic Sequencing Batch Reactors (SBR), and (iv) combined anaerobic-aerobic culture system. The integration of aerobic and anaerobic degradation pathways in a single bioreactor is capable of enhancing the overall degradation efficiency. The merits of different integrated anaerobic-aerobic bioreactors are highlighted and comparison made to identify possible future areas of research to fully utilize these methods of wastewater treatment. The comparison demonstrates that using an integrated bioreactor with stacked configuration in treating high strength industrial wastewaters is advantageous due to minimal space requirements, low capital cost and excellent COD removal efficiencies (in excess of 83%). (C) 2009 Elsevier B. V. All rights reserved.