The aim of the book is to provide students and other readers with deeper understanding of the conditions for and the nature of life in the Baltic Sea, as well as to supply knowledge on the functioning of and anthropogenic threats to the Baltic Sea ecosystem. This book is the first comprehensive science-based textbook on life in the Baltic Sea, one of the world’s major brackish water bodies. It highlights the special nature of biological variation along the unique environmental gradients of salinity and climate in the Baltic Sea area. The book treats adaptive challenges for life processes and ecosystem functioning resulting from a highly variable recent geological history, geographical isolation and heavy anthropogenic pressure. It describes and explains interactions between organisms and their environment that are central to ecosystem processes such as biogeochemical cycles, speciation and genetic diversity, patterns of species diversity, physiological adaptations, biological invasions and ecotoxicological responses. The subsystems of the Baltic Sea: pelagic zone, ice, deep soft bottoms, sandy shores, rocky shores and estuaries and coastal lagoons, are treated in detail with respect to structure, function and consequences of natural and anthropogenic changes, both global and regional. Finally, it discusses ecosystem-based management and ways to deal with contemporary and emerging threats to ecosystem health.