An analysis has been made of the uncenainty of input parameters to detenninistic models, with emphasis on the models for the sewer system. For each parameter a sensitivity analysis and for all the parameters a Monte Carlo analysis has been made. The analysis reveals a very significant uncenainty that can be decreased; but cannot be eliminated as significant to engineering application. Stochastic models have a potential for dealing with these uncertainties. That applies also to the treatment plant and to the receiving water models. The three components in the system have been analysed as a whole. Examples show that an integrated analysis is required in order to find economically optimised solutions that comply with environmental standards.