There is more than one reason to present a new key book on amphipods for the Baltic Sea. First, it is an attempt to consider all marine species observed at least once in the whole catchment area. Several species are figured and described comprehensively for the first time here. Second, due to the rapid increase of non-indigenous species, we included species that seem to be in the process of becoming established as well as those that already are. Third, the present book combines the freshwater and marine amphipod inventory of the investigation area and of some adjacent regions. The reader now has the opportunity to rely on one monograph rather than having to combine several well-known handbooks from neighbouring areas. Altogether 243 amphipod species (190 marine and 53 freshwater species) are listed and in most instances comprehensively pictured and described. On 553 plates with about 4000 single drawings the species are illustrated. The present book covers species of three different amphipod suborders. The suborders are Gammaridea (27 families, 93 species), Senticaudata (27 families, 146 species and Hyperiidea (1 family, 4 species).