The primary objective of the fourth meeting of the NATO/CCMS Expert Panel on Air Pollution Modeling was to stimulate maximum informal exchange of technical information on several topics currently considered to be of major interest and importance in air quality simulation modeling. This document provides a brief summary of the several workshop discussions and the introductory remarks of the chairmen of the sessions, together with some papers that were prepared as back-up information for the workshops. The workshops and papers addressed these topics: Applications of modeling and users' needs, validation of air quality simulation models, role of regional air pollution studies in model development, the role of empirical statistical modeling of air quality, and simplicity versus sophistication in air quality modeling.