Immersed fixed bed bioreactors are important alternatives to conventional activated sludge systems for upgrading or rebuilding wastewater treatment plants. Based on experiments in pilot and full scale, design guidelines have been established and summarized for immersed fixed beds for the removal of organic matter, nitrification and denitrification. In all applications the biochemical transfer rates are comparatively high and turn fixed beds into space saving reactors. The most important fixed bed systems such as Biocarbon, Biostyr, Biofor and Biopur are discussed with respect to their different biomass carriers, water rates, air rates, and backwash procedures. Present design information is given for different fixed beds for different nutrient removal applications. Evaluation of advantages and drawbacks of the currently offered fixed beds are still based on narrow full scale experience. In each case of potential application, the systems have to be evaluated individually on the basis of local circumstances.