The ITM conference series has always had a strong spirit of cooperation under the NATO/CCMS umbrella, and with the considerable interest fram Partner countries to participate in the ITM conferences, it provides an excellent opportunity to create ties between scientists. Whereas all previous ITM conferences have taken place in NATO countries, the 23rd ITM takes place in a Cooperative Partner country, Bulgaria, and is hosted by the National Institute of Meteorology and Hydrology, Bulgarian Academy of Sciences. This fact reflects a general wish for a closer connection and collaboration among scientists fram Partner and NATO countries. This volume contains the papers from the 23rd NATO/CCMS International Technical Meetings on Air Pollution Modelling and Its Application, being held September 28 - October 2, 1998, at Riviera Holiday Club, Varna, Bulgaria. It was attended by 120 participants from 30 countries. Thanks are due to all who made it possible to plan, carry through, and follow up the meeting, and to the participants who made the conference so successful. Special thanks are due to the sponsoring institutions: ATO/CCMS EURASAP RIS0 BAS 3M NATO/CCMS - Committee on the Challenges of Modem Society EURASAP - European Association for the Science of Air Pollution RIS0 - Ris0 National Laboratory, Denmark NIMH - National Institute of Meteorology and Hydralogy, Bulgaria BAS - Bulgarian Academy of Sciences 3M Representation office, Bulgaria Prestige Business Ltd. , Bulgaria The excellent collaboration with CIM (Company for International Meetings Ltd.