Several atmospheric pollutants have short atmospheric residence times and their influences are mainly restricted to local or meso scales (less than a few hundred kilometers). Until a few decades ago, air pollution research focused on short-range problems where the maximum ground-level concentrations are observed. However, many types of air pollutants have significant lifetimes and can potentially be transported over considerable distances. It is now commonly accepted that cost-effective approaches to air pollution abatement need to integrate the effects of air pollution transport at the regional scale. An understanding of long-range transport of air pollutants in the atmosphere requires a knowledge of the relevant atmospheric dynamic and chemical processes active at the regional scale as well as the temporal and spatial distribution of emissions. Numerical modelling is the most efficient way to determine the atmospheric transport, photochemistry and deposition pathways. The book therefore discusses the physical and chemical processes that determine regional air pollution and presents the relevant modeling techniques to describe the different atmospheric processes that are active at that scale.