Current requirements on effluent quality of municipal waste water treatment plants are dominated by the need of nutrient removal, of both nitrogen and phosphorus. Nitrification is undoubtedly the key process in biological nutrient removal plants. Many existing municipal wastewater treatment concepts are based on the combined treatment of municipal and industrial waste water. With the future need on biological nitrogen removal by nitrification/denitrification the high sensitivity to inhibiting compounds of nitrifies becomes more important. Testing procedures (based on the respiration technique) which revealed the sources of inhibition (firstly within the catchment area and subsequently inside the industries concerned) were developed. With the result of these tests, risk assessment and a priority schedule are feasible. Long term effects of the inhibiting wastewaters were investigated by operating pilot plants. On the basis of operational data, the variation in the inhibition of the maximum specific growth rate of the autotrophic microorganisms was evaluated with a newly developed method. All investigations and obtained results showed, that constantly low effluent ammonia concentrations cannot be achieved by 'end-off-pipe' technology only. Measures at the sources have to be taken in order to reduce the inhibitory effects. For the design of the extension of existing plants for nitrification and nitrogen removal, nitrification inhibition has to be taken into consideration, also when the extent of nitrification has been reduced. In many cases the concept of combined treatment could be maintained.