In 1969 the North Atlantic Treaty Organization (NATO) established the Committee on Chal- lenges of Modern Society (CCMS). The subject of air pollution was from t, lw start one of the priority problems under study within the framework of various pilot. studi('s 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 tudy relatillg to air pollution. The first five international conferences were organized by the United Statcs as the pilot country; the second five by the Federal Republic of Germany: thc third five by ßelgium; the next four by The Netherlands; ami the two most recent by Delllllark. This volume contains the papers presented at the 21st NATOjCCMS International Technical Meeting (ITM) on Air Pollution Modelling and Its Application, held in ßaltimorc. Maryland, during November 6-10, 1995. This ITM was jointly organized by the Riso National Laboratory of Denmark (Pilot Country); by the EPA National Exposurp Research Labora- tory, United States (Host Country); and by thc American MNeorologiea.! Soci('t.: -. Cnited States (Host Organization). The ITM was attended by 137 participants representing 29 countri(' from North and South America, Europe, Asia and Australia. The members of t.hc NATO/CCMS Scientific Committee for the 21st ITM were R.M. van Aalst (The Nethcrlallds), D. Allfossi (Haly), C. Borrego (Portugal), N. Chaumerliac (Francc), H. van Dop (Thc: \Tethcrla.nds). S.-E. Gryning.