The present text compiles the latest research within the field of biology performed in the Baltic Sea area. The themes span from theoretical and philosophical aspects of the ecosystem concept over population and autecological studies to detailed descriptions of plant and animal physiology. Results from microcosm and mesocosm experiments as well as direct observations in field together bring insight of the special structure and function of the Baltic Sea ecosystem. How the spawning success of cod and spat are dependent of each other and environmental factors, the impact of alien species to the composition of plankton or benthic communities, the flip of phytobenthic to planktonic communities in lagoons and mechanisms triggering the change, pure descriptions of e.g. the Estonian coast and shallow off shore areas as well as strategies for the reproductive success of Fucus vesiculosus, and the influence of eutrophication of the different Baltic Sea areas and the fate of pollutants as radionucleids and PAH etc. and other themes are all discussed in the 24 original papers of this volume.