The membrane biofilm reactor (MBfR) is an emerging technology for water and wastewater treatment based on gas-supplying membranes. Unlike conventional filtration membranes, MBfRs do not separate solids from liquids. Rather, a gaseous substrate is transferred across the membrane, while a biofilm, naturally formed on the outer membrane surface, catalyzes desired reactions. MBfRs can be supplied with air or oxygen to remove carbon and nitrogen from wastewater. They can also use hydrogen for safe and cost effective autotrophic denitrification and the treatment of other oxidized contaminants such as perchlorate and bromate. Major benefits of MBfRs include low energy consumption, high gas utilization efficiencies, small footprints, and great operational versatility. The MBfR has been the subject of extensive research over the past 15 years, and the first commercial MBfR application was released in 2011. This paper will provide a comprehensive review of the MBfR, including the most current research and future needs.