Combining membrane technology with biological reactors for the treatment of municipal and industrial wastewaters has led to the development of three generic membrane processes within bioreactors : for separation and recycle of solids; for bubbleless aeration of the bioreactor; and for extraction of priority organic pollutants from hostile industrial wastewaters. Commercial aerobic and anaerobic membrane separation bioreactors already provide a small footprint alternative to conventional biological treatment methods, producing a high-quality effluent at high organic loading rates. Both the bubbleless aeration and extractive membrane bioreactors are in the development stages. The former uses gas-permeable membranes to improve the mass transfer of oxygen to the bioreactor by providing bubbleless oxygen. By using a silicone membrane process, extractive membrane bioreactors transfer organic pollutants from chemically hostile wastewaters to a nutrient medium for subsequent biodegradation. All three membrane bioreactor (MBR) processes are comparatively and critically reviewed.