Understanding urban air pollution is a prerequisite to finding effective solutions to air quality problems and for a sustainable development in the urban environment. In this book the current state of the art in urban air pollution research is presented. A major focus is on suitable air pollution modelling concepts, covering also street canyon geometries. Such models may be applied to establish source-receptor relationships in support of urban air quality management. Procedures for evaluating the performance of air pollution models are proposed, and results from field experiments and laboratory studies are shown to provide better insight into the characteristics of air pollution at the urban and local scales. The contents of this book are of a high policy relevance, given their direct connection to the formulation of improved tools for urban air quality assessments.