Book excerpt: This book collates the written contributions of the Second Conference on Air Pollution Modelling and Simulation (APMS 2001). A wide range of current topics is covered, focusing on three challenging issues: (1) the modelling issue of complex, multiphase, atmospheric chemistry; (2) the numerical issue associated with comprehensive three-dimensional chemistry-transport models; and (3) the key issues of data assimilation and inverse modelling. State-of-the art research is presented with many operational procedures applied at either forecast agencies or companies.