After 20 years of courtship, the partnership between activated sludge and membranes in the form of membrane bioreactors is proving to be an ideal marriage. The rapid transformation of membrane bioreactors from a laboratory oddity to commercial viability in the last decade has been the result of intensive research into the fundamentals of the process. Understanding how to integrate the biological process with the physical membrane process, along with other factors has improved. This has consequently led to a substantial improvement in performance. There has also been a considerable reduction in costs, due in part to decreased membrane prices and increasing competition on the international market. With over 500 membrane bioreactor plants around the world, membrane bioreactor technology has become a cost-effective and competitive wastewater treatment technology. This paper overviews recent developments in the area of membrane bioreactors and describes Bord na Móna Environmental's experience with this process, along with identifying future opportunities for the technology.