In different pilot experiments with tertiary trickling filters, rotating biological contactors, aerated biofilters and other fixed bed reactors, the effects of transient NH4-loading rates, transient oxygenation capacities, short- and long-term organic loading on nitrification performance were studied. Operating tools and mechanisms are shown to enhance NH4- and O2-limited nitrification rates. Periodic flow reversal in trickling filter and rotating contactor series and the use of pure oxygen in biofilters are possibilities to increase the nitrification capacity of tertiary biofilm reactors. Due to O2-competition and heterotrophic overgrowth, the presence of biodegradable organics may lower or completely stop the nitrification activity of tertiary biofilms. In this respect frequently backwashed biofilters and other fixed beds are superior to conventionally operated trickling filters or rotating biological contactors with stationary biofilms. Frequent removal of the top biofilm layers by backwashing is shown to offer the advantage of enhanced O2-supply into the depth of the biofilm and the possibility to maintain thin and fully aerobic multispecies biofilms with elevated nitrification potential.