The environmental history of the seas and oceans has generally remained a relatively unexplored theme. This chapter addresses the environmental history of the Baltic Sea, which is a European sea par excellence and the only sea that is entirely located within the continent. We will examine the links between wider historical currents in Europe and the marine environmental history of the Baltic Sea by focusing on three environmental regimes from the end of the nineteenth century until the 1990s. The first environmental regime was developed on an urban level and prevailed from the late nineteenth century until the Second World War. The second environmental regime was developed from the 1960s until the 1970s on an international level in the Baltic Sea region. The third environmental regime, spanning the 1980s and 1990s, consisted of developing wider European cooperation. These three different environmental regimes continue to cooperate in the region even today.