[en] Spent fuel as a high level waste form has been studied for over a decade. Although the fuel contains a multitude of radionuclides, only a few of them will actually constitute a long term potential radiotoxic hazard. These elements are, after some thousand years, plutonium and neptunium. Up to about 100,000 years plutonium is by far dominating the radiotoxicity after which neptunium becomes the dominating element. In this paper, the results obtained on plutonium and neptunium releases from spent fuel are reviewed and discussed. Attempts are made to interpret the data obtained for different fuel types and groundwaters in terms of the chemistry of the respective elements as well as in terms of release kinetics