Fish and invertebrates are subject to a wide range of disease agents. Many of their diseases are probably local in origin although a lengthy list of infections were probably imported via the vast array of exotic species which have been transferred to virtually all areas of the world. Since ail living organisms carry a full suite of microorganisms and larger parasites the likelihood of there being pathogens for local species among them is good. Introductions can occur in at least one of three separate ways: (1) intentional introductions for specific purposes, (2) accidental transport of biological agents via massive transfers of ballast waters or (3) through the ornamental or aquatic pet trade. Control measures and information services devised by the International Council for the Exploration of the Sea, the European Inland Fisheries Advisory Commission and the Office International des Epizooties are described.