In recent years, substantial attention has been paid towards the compact high-rate bioreactors for industrial and municipal wastewater treatment to meet the strict constraints with respect to space, odor, view, and biosolids production. Hence, integrated anaerobic-aerobic bioreactors which combine the aerobic and anaerobic processes in a single reactor are definitely a viable alternative to conventional anaerobic-aerobic system. This review focuses on the performances of the integrated bioreactors in terms of chemical oxygen demand (COD) removal. The integrated anaerobic–aerobic bioreactors are classified into two types: i) integrated bioreactors with physical separation of anaerobic-aerobic zone, and ii) integrated bioreactors without physical separation of anaerobic-aerobic zone. The integrated bioreactors without physical separation of anaerobic-aerobic zone that are devised with stacked configuration exhibit great potential in treating high strength industrial and municipal wastewaters attributable to their tremendous advantages such as excellent COD removal efficiencies, minimal space requirements, and low capital cost. It is envisaged that the integrated bioreactors are able to attain Certified Emission Reduction (CER) credit derived from Clean Development Mechanism (CDM) under Kyoto Protocol 1997 if methane gas capture system is properly installed.