Human activities cause plant and animal species to become introduced to areas they could not have reached by natural dispersal. A significant component of human induced loss of biodiversity is the movement of species between continents, regions and countries. This report identifies 1387 species introduced intentionally or unintentionally to the Nordic countries. Common introduction modes as well as the ecological effects of introduced species are exemplified by describing a selection of species in more detail. Furthermore the report gives an overview of the relevant international laws and guidelines. Introduced species are a threat to the native biological diversity, as well as to the human economy, and coordinated and specific actions addressing the problem are needed. This is true on a global as well as on a Nordic scale. Prevention is better than the cure, since it is almost impossible and indeed very costly to eliminate most invasive species. The report gives recommendations on legal, institutional, management, education, awareness and research needs with the goal of reducing the ecological and economical impact of introducing species to the Nordic countries.