1 Ambio, 2001 (version 2001-03-07) Giving advice on cost effective measures for a cleaner Baltic Sea: a challenge to science. Fredrik Wulff , Erik Bonsdorff , Ing-Marie Gren, Sif Johansson , Anders Stigebrandt Abstract The Baltic Sea is one of the areas of the world that is most severely affected by human activities. Although there is an international agreement that nutrient input should be reduced, the measures taken so far has not resulted in major reductions in nutrient inputs nor in environmental improvements. The reasons are partly due to lack of knowledge on large-scale relationships and couplings between physics, biogeochemistry and ecological properties. But there is also a lack of overall drainage basin wide analyzes on cost effective measures. There is a danger in making wrong decisions, e.g. implement reduction schemes that are at worst ineffective or at best, far from cost effective. Now researchers from many disciplines are faced with a common challenge: To develop a decision-support system, which can be used as the scientific base for cost-effective measures for the entire Baltic Sea. Such an effort is now made within the research program MARE (http://www.mare.su.se .