Mass balances for carbon (as COD), nitrogen, and phosphorus are excellent tools for the evaluation of WWTPs in order to estimate fluxes of these substances, to compare operational data of different WWTPs with one another and, finally, to draw conclusions of general validity. In the case of activated sludge systems, mass balances can also be very helpful for model calibration, in particular for the ‘Activated Sludge Model No. 1’ (ASM1), because this model is consequently based on balances for COD and nitrogen. Hence, the results of mass balances can be utilised directly for the estimation of model parameters. Accordingly, it is recommended to evaluate the fluxes of COD and nitrogen, before the model calibration is started. By means of mass balances only the state of equilibrium can be described. Therefore, as regards model calibration, only parameters responsible for long-term behaviour can be estimated, i.e. the heterotrophic yield YH, the fraction of biomass yielding particulate organics fP, the heterotrophic decay rate bH and the concentration of particulate inert organics XI in the influent. A correct estimate of these parameters is a prerequisite for the validity of the simulation results, not only in regard to excess sludge production and SRT, but also with respect to the oxygen consumption for carbon removal and, therefore, to denitrification capacity.