In 2007 HELCOM launched a plan for transboundary management of the Baltic Sea. This plan, called the Baltic Sea Action Plan (BSAP), aims amongst other things, to reduce eutrophication in the different regions of the Baltic Sea by reducing incoming nutrient loads from all discharging drainage basins. This paper investigates the costeffective distribution of nutrient abatement measures between drainage basins and Baltic Sea regions, where the aim is to achieve the BSAP nutrient load reduction targets. The cost-minimisation model BALTCOST, an interdisciplinary development involving economists and natural scientists, is used to identify the most cost-effective configuration of abatement measures. BALTCOST utilises detailed regional and spatial data down to 10 x 10km grid cell level for all Baltic littoral countries. Modelling results suggest that it should be possible to achieve the BSAP load reduction targets for N and P in most Baltic Sea regions, with the exception of the P load target in the Baltic Proper and the N reduction target in the Danish Straits, where only 74% and 88% respectively, of the desired BSAP load reductions can be delivered. BALTCOST results indicate that the BSAP targets appear difficult to achieve, and that additional abatement measures are likely to be required to fulfil these targets. The minimised total cost of delivering the achievable load reductions across the 9 Baltic littoral countries is estimated to be 4.69 billion Euros, annually, with substantial differences in the cost burden between the countries. Comparison of N and P reductions at source and at sea shows that groundwater and surface water nutrient retention is highly influential over the costs incurred in fulfilling the load reduction targets. Disregarding spatial variation in nutrient retention is clearly unrealistic, and the detailed modelling of nutrient retention, together with the spatially detailed dataset behind the cost-estimates, are novel features of this study; previous studies have used more aggregated estimates of retention and regional costs.