A general consensus has been reached among technical experts that high-level radioactive waste can safely be disposed of in deep geological repositories. Safety studies are carried out to evaluate the overall capacity of a particular disposal site to confine waste and minimise radioactive releases. Since the principal way in which radioactive elements might eventually reach the biosphere is by transport of dissolved radionuclides in groundwater, the safety study calculations must be able to estimate their rate of transfer through each of the barriers surrounding the repository. It is well known that, for many radioelements, sorption reactions can lead to a reduction of the amount of radionuclides present in the solution phase. How best to take radionuclide sorption reactions into account in repository performance assessment models is the subject of this book.