Wastewater systems consist of sewer systems and wastewater treatment works. As the performance of a wastewater treatment plant is affected by the characteristics, i.e. operation and design, of the contributing sewer systems, knowledge of the interactions between sewers and wastewater treatment works is very important when trying to understand and improve wastewater systems. The thesis comprises three main topics: Firstly, the sensitivity of wastewater treatment plant performance to influent fluctuations is discussed, based on both measurement data and model results. Secondly, the quality of model results achievable with state of the art sewer models are described for the most important in sewer processes. Finally, the impact of the interactions within the wastewater system on wastewater system performance is illustrated in a semi-hypothetical case study. The thesis concludes that knowledge on the interactions is not only essential with respect to understanding of wastewater system performance, but that this knowledge opens new ways of dealing with our existing wastewater infrastructure.