Membrane bioreactors (MBRs) are composed of activated sludge processes and membrane filtration. Because of their unique advantages such as good effluent quality and compact structure, MBRs have been widely used for municipal and industrial wastewater treatment. Recent advances in MBR research are reviewed by focusing on development of bioreactor configurations, enhanced degradation of pollutants, and sludge reduction. Efforts of a number of novel MBR processes such as hybrid biofilm MBR, submerged rotating MBR, MBR with reverse osmosis, osmotic MBR, membrane distillation bioreactor, air-sparging MBR, and jet loop MBR for the treatment of nitrogen, phosphorous, emerging contaminants, heavy metals, and sludge reduction are summarized. Process principles, benefits, and limitations of these MBRs are discussed. According to a detailed analysis of research publications, MBR research has undergone an extensive growth in the areas of development of novel MBR configurations and application of MBRs with new purposes.