Since the construction of wastewater treatment plants combined sewer overflows have become an increasingly important limitation to the quality of the surrounding surface waters. Over the years urban water resources have often been so modified by anthropogenic activity that water quality management requires an integrated approach both at an evaluation and an investment level. Effects of acute pollutants, such as oxygen depletion caused by excessive organic material, should be evaluated on the basis of their return periods. Models of the relevant water systems are then used both to calculate the present return periods and to predict those of proposed amelioration projects. In traditional deterministic modelling of combined sewer systems there has been a tendency to continuously add new processes to the model structure in an attempt to improve the quality of the predictions made. This has often resulted in many model parameters with unknown values and the inclusion of processes much less significant than others that are not known or well understood. An alternative approach is to describe only the most essential processes and to include stochastic terms to describe the remaining variation. The present study has focused on comparing and contrasting deterministic and stochastic approaches to modelling of urban runoff pollution and water quality in general. Methodologies surrounding the application of models in return period analysis and its uncertainty have also been studied.A multivariate analysis was made on event mean concentrations data sets from three Dutch and two Danish combined sewer catchments (Chapter 3). This was done to examine the underlying structure of variations in event mean concentrations. Results confirmed expectations that the most pronounced common variations relate to the groups of particulate pollutants and dissolved pollutants. The distribution of the principal factors clearly reconfirmed the bimodal or mixed distribution that have earlier been reported for event mean concentrations of ...