This work treats 157 species of predaceous diving beetles (Dytiscidae) found in Finland, East Fennoscandia, Sweden, Norway, Denmark and the Baltic State. Introducing chapters provide general information about the state of knowledge and distribution, morphology of adults and larvae, bionomics, collecting and rearing. The main part of the text comprises keys and descriptions for identification of adult beetles, including information on distribution and biology of all species. A catalogue shows provincial records in Fennoscandia and Denmark. This work is based on recent advances in synonymy and taxonomy of the family, and on the study of thousands of specimens in collections and field.