Assessments of pollutant effects on living organisms usually have used dose-response relationships (models) based on data for one pollutant and one species at a time, for a relatively short period of exposure, under relatively controlled conditions. Such studies are necessary, but even literature reviews of the results provide only limited insight into the response of regional resource systems where there are multiple pollutant impacts. For the assessment planned under the 1980 U.S./Canada Memorandum of Intent on long-distance pollutants a Work Group was established specifically to evaluate effects from the apparent elevated levels of pollutants transported long distances. This paper reports on the use of both qualitative comprehensive models and quantitative empirical models as methodologies for the Aquatic Impacts Assessment process in Phases I and II of that study.