In 1969 the North Atlantic Treaty Organization (NATO) established the Committee on Challenges of Modern Society (CCMS). The subject of air pollution was, from the start, one of the priority problems under study within the framework of various pilot studies undertaken by this committee. The organization of a periodic conference dealing with air pollution modelling and its application has become one of the main activities within the pilot study relating to air pollution. This volume contains the papers and poster abstracts presented at the 25th NATO/CCMS International Technical Meeting on Air Pollution Modelling and Its Application held in Louvain-la-Neuve, Belgium, during 15-19 October 2001. This ITM was jointly organized by the University of Aveiro, Portugal (pilot country) and by the Catholic University of Louvain, Belgium (host country). The main topics of this ITM were: the role of atmospheric models in air pollution policy and abatement strategies; integrated regional modelling; global and long-range transport; regional air pollution and climate; new developments; and model assessment and verification. Invited papers were presented by A. Ebel of Germany (Changing atmospheric environment, changing views); H. Simon of Great Britain (Applying risk assessment techniques to air pollution modelling and abatement strategies); and S.E. Gryning of Denmark (Aspects of meteorological pre-processing of fluxes over inhomogeneous terrain) - all of which are included in this comprehensive proceedings volume.