[en] Long-term isolation of radioactive waste can be provided by a combination of natural and man-made barriers. Geological formations of different types have been proposed as suitable natural barriers for radioactive waste isolation. Argillaceous formations can have very favourable characteristics such as: low permeability, high sorption capacity, high plasticity. The retention properties and the low intrinsic permeability can guarantee waste isolation as long as the rock is homogeneous and integer. The presence of undetected fractures, or fractures formed after waste emplacement, can seriously compromise the efficiency of the barrier. In general terms clay formations are expected to respond in a plastic way to external stresses, thus fault and fracture formation should be a rare phenomenon in such media. This plastic behaviour seems supported by mathematical models applied to both conceptual and specific cases. Nevertheless, faults and fractures have been observed in clay outcrops, in quarries and in relatively deep tunnels. In some quarries, fissures surrounded by oxidation zones 1 or 2 cm thick have been seen. The mechanisms of fault and fracture formation in clays must be investigated and the possibility that they can act as pathways of enhanced water and radionuclide migration must be evaluated. (Auth.)