Membrane bioreactors can be defined as systems combining biodegradation of waste products with filtration. These are very effective in removing organic and inorganic contaminants as well as biological entities from wastewater. Main advantages of the MBR include good control of biological activity, high quality effluent free of bacteria and pathogens, smaller plant size, and higher organic load rates. This article aims to review all the principles and potential applications of the MBR technology.