The Baltic Sea, the heart of the Baltic region, is one of the most polluted seas worldwide. For the countries of the Baltic region, the relative importance of the Baltic Sea varies, but all coastal states of the Baltic Sea use the sea and influence it through their manifold activities. The protection of the Baltic Sea therefore is a shared concern for the coastal states. This shared concern has led to the emergence of a specific international legal régime governing the Baltic Sea. In this text, current threats to the Baltic Sea's natural environment and the international legal measures that are taken to protect the sea are described, in particular with a view to possible improvements. Particular emphasis will be placed on the northernmost part of the Baltic Sea, the sub-Arctic Bay of Bothnia that faces particular environmental challenges.