We present an updated,harmonized hydrologic basemap of the entire Baltic Sea Drainage Basin (BSDB),including 634 subdrainage basins.The updated map hasa level of detail approximately 5 to 10 times higher thanthe current standard and includes various spatial-aggre-gation possibilities of relevance for water management.All 634 subdrainage basins and their various spatialaggregations are characterized in terms of population,land cover,drainage density,and slope.We identify,quantify,and characterize,in particular,drainage basinsthat are unmonitored with regard to the combination ofwater-flow and nutrient-concentration measurementsneeded to monitor coastal nutrient and pollutant loading.Results indicate that out of a total BSDB population of84 239 000 in 2002,24% lived in unmonitored coastaldrainage basins that cover 13% of the total BSDB area.Amore detailed analysis of Swedish catchments indicatesthat Sweden has a particularly large proportion of un-monitored coastal catchment areas (20% of the totalSwedish area) with high population pressures (55% of thetotal Swedish population),when compared with averageconditions for the whole BSDB.in general,the investi-gated characteristics of unmonitored coastal basins varyand differ largely from those in adjacent monitoreddrainage basins within the BSDB. We present an updated, harmonized hydrologic basemap of the entire Baltic Sea Drainage Basin (BSDB), including 634 subdrainage basins. The updated map has a level of detail approximately 5 to 10 times higher thanthe current standard and includes various spatial-aggre-gation possibilities of relevance for water management. All 634 subdrainage basins and their various spatialaggregations are characterized in terms of population, land cover, drainage density, and slope. We identify, quantify, and characterize, in particular, drainage basinsthat are unmonitored with regard to the combination ofwater -flow and nutrient-concentration measurements of to monitor coastal nutrient and pollutant loading. Results indicate that out of a total BSDB population of 84,239,000 in 2002, 24% lived in unmonitored coastal drainage basins that cover 13% of the total BSDB area. More detailed analysis of Swedish catchments indicatesthat Sweden has a particularly large proportion of un-monitored coastal catchment areas (20% o f the totalSwedish area) with high population pressures (55% of the total Swedish population), when compared with average conditions for the whole BSDB. in general, the investi-gated characteristics of unmonitored coastal basins vary and differ largely from those in neighboring monitored drainage basins within the BSDB.