This chapter investigates the current state of knowledge on engineered nanomaterials (ENMs) and their behaviour in wastewater treatment processes in order to identify areas for future research. It focuses on the processes currently in use for urban sewerage treatment and begins by investigating the presence of engineered nanomaterials in wastewater treatment plants. It the moves on to examine the possible retention and aggregation of engineered nanomaterials in activated sludge and explores the possible transformations that ENMs can undergo in treatment plants and the models that are available to predict these transformations. The chapter also discusses the potential risks of agricultural application of sewerage sludge that is charged with engineered nanomaterials. The chapter concludes by identifying knowledge gaps and areas where additional research would be required.