The report summarizes the results of an EPA-sponsored workshop that was held to discuss procedures and protocols for evaluating regional-scale acid deposition models. The workshop was the first of three that are planned to assist the U.S. Environmental Protection Agency, the Ontario Ministry of Environment, the Atmospheric Environment Service of Canada, and the Electric Power Research Institute in designing a model evaluation program. The workshop was asked to consider four major topics: (1) procedures to be used in evaluating the performance of acid deposition models and methodologies for applying these procedures; (2) data requirements of these procedures and methodologies; (3) the probable impact of time and budget constraints on the evaluation process; and (4) possible conflicts between client needs and the probable output of the evaluation program.