Over the past 50 years, explosions of nuclear weapons and releases from the nuclear industry have introduced anthropogenic plutonium into the environment. In the Arctic environment, the main source of plutonium is from atmospheric weapons testing, but studies of plutonium in the Kara Sea have shown that other releases have given rise to enhanced local concentrations. Since different plutonium sources are characterised by distinctive plutonium-isotope ratios, evidence of a localised influence can be supported by clear perturbations in the plutonium-isotope ratio fingerprints. To assess long-term consequences when radionuclides are released into the environment, information on the source term speciation, transport and transformation processes and biological uptake is needed. Among artificial radionuclides released to the environment, the transuranium elements are a major concern, due to very long half-lives and their accumulation in bone as well as high radiotoxicity. Plutonium has been produced in greater quantity than other transuranic elements, however, environmental assessments are complicated by the complex environmental behaviour.