The combination of membrane separation technology and bioreactors has lead to a new focus on wastewater treatment. The application of membranes has led to very compact wastewater treatment systems with an excellent effluent quality. For concentrated wastewaters, like industrial streams and landfill leachate the membrane bioreactor has been applied at full scale successfully. The relatively high energy requirements have hindered the wide spread application of membrane bioreactors. Using new membrane techniques, like transfer flow modules, creates the possibilities of a more widespread application. This opens possibilities for far going reuse of wastewater, both industrial and municipal, decrease in sludge production and small-footprint bioreactors for less concentrated wastewater streams.