Membrane aerated biofilm reactors (MABRs) represent a relatively new biotreatment technology. In a MABR, biofilm is grown on a gas-permeable membrane (often a hollow fiber membrane). Soluble organic compounds in the liquid are supplied to the biofilm from the biofilm-liquid interface whereas oxygen supply to the biofilm is from the biofilm-membrane interface (by oxygen diffusing through the membrane). MABRs can achieve bubble-less aeration and high oxygen utilization efficiency (up to 100%) and the biofilm can be stratified into aerobic/anoxic/anaerobic zones to simultaneously achieve removal of carbonaceous organic pollutants as well as nitrification and denitrification (if needed) in a single biofilm. This article briefly reviews the MABR process, including the characteristics, membrane materials, modular design, operation parameters and the potential applications, from relevant recent patents and literature.