Thanks to undeniable advantages as recovery of energy and a low sludge production, anaerobic wastewater treatment has received recognition in the last decade. However due to the slow methanogenic growth, inreactor biomass accumulation is essential to maintain high loading rates. Indeed wash-out of biomass is one of the main problems encountered in the anaerobic treatment of industrial effluents. A broad scoped overview of the different reactor technologies is given each with its proper biomass accumulation system. All of them are founded on two basic mechanisms for biomass accumulation: separation and retention. Settling (Anaerobic Contact reactor), ultrafiltration and flotation (FlotametP) are the techniques used to separate externally the biomass from the effluent after which the biomass can be recirculated to the anaerobic reactor. Concentration of biomass can also be achieved by attachment to a mobile (Fluidized Bed reactor) or a static carrier with possible entrapment in its macroporous structure (Anaerobic Filter reactor). The UASB-reactor incorporates retention as well as separation. By stimulating granular growth, biomass is accumulated in the lower part of the reactor. A three phase separator at the top of the reactor contributes to the final clarification of the effluent. Hybrid reactors eventually combine different biomass accumulation mechanisms: granulation, external biomass separation (Upflow Anaerobic Contact reactor) and fixation (Upflow Anaerobic Contact Filter reactor).