The overall focus of the book is on knowledge about the conditions for life in brackish water and the functioning of the Baltic Sea ecosystem. It highlights biological variation along the unique environmental gradients of the brackish Baltic Sea Area (the Baltic Sea, Belt Sea and Kattegat), especially those of salinity and climate. Throughout the book, we follow an ecosystem approach. Rather than focus on separate topics regarding e.g. plankton, fish or birds, or environmental issues such as eutrophication or fisheries, the chapters focus on processes and subsystems. Ecological compartments and environmental issues are treated jointly with the processes and subsystems when relevant. Some overlap between chapters has been allowed when appropriate, e.g. with respect to an issue viewed from different angles or in a different context.