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As a consequence of the awareness of the pollution impact of storm sewer overflows, managers need tools to evaluate and control stormwaters according to water quality criteria. After an experience of 25 years in storm water quality modelling, very few models are widely and regularly used. According to managers this is due to their cost and their low level of accuracy. The generation and the transport of the pollution in urban systems during a storm event are very complex because they concern many media and many space and time scales. Nevertheless, a typology of the existing models shows that this complexity has been inscribed into the models. This tendency towards complexity makes sewer quality models difficult to put into operation and three main difficulties can be underlined: doubtful mathematical formulation of processes, uncertainties on input and calibration data, difficulties and cost of calibration …