This publication summarizes the results of a Coordinated Research Project carried out by the IAEA’s Marine Environment Laboratory in Monaco. The results obtained confirm that the dominant source of anthropogenic radionuclides in the marine environment is global fallout; however, important contributions have also been due to authorized releases of radionuclides to the marine environment from the Sellafield and Cap de la Hague reprocessing plants, as well as from the Chernobyl accident. Time trends in radionuclide concentrations in surface water were studied and radionuclide mean residence times in the world oceans were estimated. Similar mean residence times were obtained for 90Sr and 137Cs, 28 ± 3 years, and 13 ± 1 year for 239,240Pu. The results provide the most complete data set available to Member States on levels of anthropogenic radionuclides in the marine environment. They are used as the international reference source on the average levels of anthropogenic radionuclides in the marine environment, so that any further contributions from nuclear reprocessing plants, radioactive waste disposal sites, nuclear bomb test sites and possible nuclear accidents can be identified.