Anaerobic wastewater treatment is an attractive and generally accepted technology for the treatment of various types of medium- and high-strength wastewaters. So far, this treatment technology is mostly applied at the mesophilic temperature range between 25 and 40°C. However, results of recent research conducted under both psychrophilic (< 20°C) and thermophilic (> 45°C) conditions, reveal that temperature is not a limiting factor in applying anaerobic treatment, provided the appropriate process design is chosen. Temperature has a considerable impact on various biological and physical factors of the anaerobic conversion process. For instance, the biogas production rate is reduced to a minimum at low temperatures, while it can reach extreme values under thermophilic conditions. In sludge bed systems, the biogas production rate determines the degree of mixing between the biomass and the wastewater and should, therefore, be considered in the process design. Other impacts of temperature are related to inhibition effects under thermophilic conditions and to a non-desirable accumulation of non- or partly degradable organic matter under psychrophilic conditions. Obviously, these effects may hamper the utility of the commonly applied single stage reactor systems. However, by adapting the process design to the expected prevailing conditions inside the reactor, the loading potentials and overall stability of the anaerobic high-rate process may be distinctly improved.