"INTRODUCTION Under contract to the American Petroleum Institute@ Battelle@ Pacific Northwest Laboratories conducted a review and evaluation of mathematical models for the prediction of photochemical secondary pollutants. The final report for this project summarizes the present and future roles of photochemical models in air quality decisions. In addition@ the report describes the deficiencies in current research needs@ and discusses future modeling and computational trends. The report describes the physical and chemical problems involved in following the precursor components of photochemical smog from emissions to the final products of the oxidation process. In addition@ it outlines the various features in visibility impairments due to airborne primary and secondary aerosols and gaseous species such as NO2 and H2O. These adverse atmospheric phenomena are organized@ described@ and explained in terms of simulation models. The report is written in the spirit of a ""model appreciation"" course for those interested in photochemical reactions of airborne pollutants. The intended user of the report is not the working photochemical modeler@ but technical managers or project leaders responsible for writing environmental impact statements or satisfying environmental control. It is felt that the report will aid a manager's staff in implementing a photochemical model for a given application or lend support to a decision to use external consultants for exercising a complex photochonical model in a given situation."