The Baltic Sea area is known to host about one hundred aquatic alien species. An alien species is any species intentionally or accidentally transported and released by man into an environment outside its native geographical range of habitat. These human-mediated invasions represent a growing problem due to the unexpected and harmful impacts the alien species cause to the environment, economy and human health. The Baltic Sea Alien Species Database is seen as a regional node in the future Global Information System for Invasive Species. The goals of the Database are: -to provide a qualified reference system on alien species for the Baltic Sea area, available online for environmental managers, researchers, students and all concerned; -to update the information on the Baltic Sea alien species, their biology, vectors of introduction, spread, impacts on environment and economy through the online questionnaire involving the data flow from research institutions and responsible environmental authorities; -to encourage the exchange of data among different geographical regions and thereby to serve a node in the Global Information System for Invasive Species.