This book describes the main concepts used to develop and implement chemistry-transport models to calculate the evolution of regional air pollution. Since physico-chemical principles are already widely presented in various works, the perspective chosen for this book concerns the modeling of these processes. As modeling can be a simplification of reality in a particular study framework, we will try to show whether the processes represented are well modeled or not. For each process, we will discuss the simplifying assumptions that have been made, the various possible ways for improvement and the impact of these simplifications on the desired results. General information on pollution is presented, followed by observations, legislation, modeling of meteorology and then chemistry-transport, anthropogenic and natural emissions, depots, validation of calculations, optimization and data assimilation.