The book "Biology of the Baltic Sea" offers a comprehensive compilation of the latest research findings in the field of Baltic Sea biology. It covers a wide range of topics, from theoretical and philosophical aspects of the ecosystem concept to detailed studies on population and autecology. The authors present results from microcosm and mesocosm experiments as well as direct field observations, providing valuable insights into the unique structure and function of the Baltic Sea ecosystem. Topics discussed include the dependence of reproductive success in cod and shellfish on environmental factors, the impacts of invasive species on the composition of plankton and benthic communities, and the mechanisms that trigger the transition from phytobenthic to planktonic communities in lagoons. Additionally, strategies for the reproductive success of Fucus vesiculosus and the influences of eutrophication on various areas of the Baltic Sea are discussed.