Very high levels of air pollution have been observed in many cities across the world, in both developing and developed economies, with varying degrees of regularity. Predicting urban air quality demands detailed knowledge of both the physical properties of the urban atmosphere and pollutants within it, and the chemical reactions of those pollutants, which have a major impact on measured levels. For emitted pollutants, concentrations are likely to be reduced proportionately with reductions in emissions, but in the case of secondary pollutants formed within the atmosphere, the relationship between precursor emissions and reaction products is often strongly non-linear. This discussion aims to improve understanding of the underlying processes responsible, which is essential for the development of high quality numerical models of urban air pollutants, which are required for the testing of mitigation strategies prior to implementation. The following topics are covered within this volume: Current status and trends in air quality in megacities Physico-chemical processes in the urban atmosphere (neighbourhood scale) Physico-chemical processes in the urban atmosphere (city scale) Effects, mitigation and policy