The Baltic Sea is almost totally surrounded by land and therefore more endangered by pollution than other marine areas. The sources of marine pollution are municipal and industrial waste inputs directly into the sea or via rivers, and atmospheric inputs mainly from traffic and agriculture. The increase of inorganic plant nutrients (NH3, NOx, PO4) caused eutrophication and consequent oxygen depletion in coastal bottom waters as well as in the depths of the open sea. In the anoxic sediments, hydrogen sulfide can be produced by protein-decomposing and sulfate-reducing bacteria. The bottom fauna will be destroyed and only H2S tolerant microorganisms can survive. Originating from cellulose manufacturing and from paper mills, large amounts of poisonous chlorinated compounds contaminated the coastal waters of Sweden and Finland until the 1980s. Most of this material is still present in sediments of the central Baltic Sea and can be resuspended by near bottom currents. To reduce pollution and improve the situation in the Baltic Sea, the surrounding countries organized the Helsinki Convention, which came into force on 3.5.1980. The Helsinki Commission (HELCOM) founded in 1974 acts as coordinator and is responsible for the enforcement of the Baltic monitoring program and international research projects. The activities of HELCOM have led to the reduction of dangerous pollutants which in turn has caused the regeneration of flora and fauna in some areas. Further improvements can be expected.