Natural radioactivity provides tracers in a wide range of characteristic time scales and reactivities, which can be used as tools to study the rate of reaction and transport processes in the ocean. Apart from cosmogenic nuclides and the long-lived radioisotope K-40, the natural radioactivity in the ocean is primarily derived from the decay series of three radionuclides that were produced in the period of nucleosynthesis preceding the birth of our solar system: Uranium-238, Thorium-232 and Uranium-235 (a fourth series, including Uranium-233, has already decayed away). The remaining activity of these so-called primordial nuclides in the EarthŽs crust, and the range of half lives and reactivities of the elements in their decay schemes, control the present distribution of U-series nuclides in the ocean.