"Why is the Baltic Sea so special to live in", is the main question the authors here give several arguments or answers for. Geographical position, geological development, hydrographical features, climate and physical drivers together create the Baltic Sea environment. The Baltic Sea water is brackish and characterized by pronounced salinity gradients, both in horizontal and vertical directions, because of the large volume of freshwater runoff from over 100 rivers, which mixes with the saline water from the Kattegat that enters the Baltic Sea via narrow shallow straits. Being a semi-enclosed continental sea with a large drainage area compared to its water volume , the Baltic Sea ecosystem is heavily impacted by the surrounding landmasses. The water residence time in the Baltic Sea is long (30–40 years), and therefore discharged nutrients and toxic compounds circulate within the sea for a long time, which contributes to its vulnerability to eutrophication and chemical contamination by hazardous substances. The Baltic Sea Area is geologically young and the Baltic Sea ecosystem is extremely young in an evolutionary perspective. Only few macroscopic species are fully adapted to its low-salinity environment. In an ecosystem-wide perspective, the large-scale Baltic Sea gradient is the principal ecological characteristic of the Baltic Sea.