This paper reviews the development and evolution of commonly used anaerobic reactors - anaerobic filters (AF), anaerobic fluidized bed reactors, upflow anaerobic sludge blanket (UASB) reactor - for wastewater treatment. The successful application of anaerobic technology to the treatment of industrial wastewater is critically dependent on the development and type of anaerobic reactor used. Since the original design was developed, many codifications have been made in reactor design in order to enhance both the efficiency and reliability of the reactor. In this paper, the ain alteration and modifications of anaerobic reactor would be documented.