A primary objective of the study is to improve the understanding of the role of performance quality in determining a model's acceptability and usefulness to the policy maker, and thus to aid in developing soundly-based expectations of the modeling process. The vehicle for pursuing this objective is examining the historical evolution of the Urban Airshed Model (UAM), a grid-based photochemical model whose basic formulation is similar to the Regional Acid Deposition Model, and its application to policy analysis in the South Coast Air Basin of California. A derivative objective is to draw implications from the findings to aid in appraising the merits of future pursuits, notably the mounting of comprehensive field programs to support the evaluation of regional acid deposition models. The report describes the UAM, examines its predictive capability through scrutiny of historical performance statistics, assesses its degree of acceptance in the scientific and regulatory communities, based on information and viewpoints solicited through questionnaires and interviews, examines the influence on the policy-making community of the UAM studies and, based on these 'findings', attempts to develop a perspective on the expectations of current regional acid deposition models.