In this chapter we examine the air pollutant emissions and atmospheric dispersion processes which drive urban air quality. Road transport is a major source of urban air pollution but there are many other stationary sources ranging from industrial plants and domestic machinery. The three main urban air pollutants are nitrogen oxides (NOx), ozone (O3) and particulate matter (PM), the latter two being partly transported from other regions and countries. Air quality models are used, not only to predict air pollutant concentrations but also to assess the most cost- effective solutions to improve urban air quality. We concentrate on how pollution from road transport can be reduced over the next two decades in Europe and Asia. A new win–win fuel for heavy duty vehicles will be hydrogen which can be readily manufactured from renewable energy sources. Urban air pollution will become a problem of the past by 2050 with zero emissions road transport and greater use of rail transport.