This interdisciplinary paper reports the results of a study into the costs and benefits of eutrophication reduction in the Baltic Sea. A large multidisciplinary team of natural and social scientists estimated nutrient loadings and pathways within the entire Baltic drainage basin, together with the costs of a range of abatement options and strategies. The abatement cost results were compared with clean-up benefits on a basin-wide scale, in order to explore the potential for international agreements among the countries which border the Baltic. Most countries would seem to gain net economic benefits from the simulated 50% nitrogen and phosphorus reduction policy.