The application of Membrane Bioreactor (MBR) in waste water treatment is widespread. The Membrane Bioreactor process consists of a suspended growth biological reactor combined with a membrane unit located either external to the bioreactor or mounted within the bioreactor. Nowadays researchers have been exploring the use and extend of their applications for treatment of municipal and industrial waste waters. It is an efficient process for maintaining a long solids retention time(SRT) at a relatively short hydraulic retention time(HRT).While a high solids retention time is desirable for process stability, a short hydraulic retention time reduces the reactor volume and hence reduces the capital costs. Domestic as well as industrial waste water is commonly performed in activated sludge plants consisting of a biological reactor for the degradation of pollutants and a settler for the separation of the sludge and purified water. Newer concepts (MBR) substitute the settler by a submerged membrane separation in the aerated biological reactor. The advent of membranes makes the wastewater treatment easier nowadays. Activated sludge process (ASP), which is the oldest technique in waste water treatment, is combined with highly efficient membrane filtration to start a sophisticated technique called Membrane Bioreactors (MBR). It is an efficient process for maintaining a long solids retention times (SRT) at a relatively short hydraulic retention time (HRT), which is needed for the treatment of wastewater. MBR is favoured to all other conventional techniques because the treated water is free from suspended solids and microorganisms, thus making it suitable for reuse. This unique application gives high degradation rates, extremely low sludge production and very compact design. The seminars includes the conventional activated sludge process, description of membrane bioreactor operation, various types of filtration that are used in it, its design parameters and applications. It also gives a brief description of cleaning of the membrane.