Guidelines have been drawn up for the selection of possible sites for high-level radioactive waste disposal in or on the seabed, which were prepared for the U.K. Department of the Environment. The first step in producing the guidelines was to identify potential failure modes for each disposal option. The guidelines were then developed on the basis of minimizing such failures. No detailed attempt has been made to rank the guidelines, since a proper evaluation of any disposal site must include an analysis of all the interdependent components of the disposal system. However, for disposal within the seabed, the main emphasis is on the geological stability and barrier properties of the disposal medium (the seabed rocks or sediments) and on the engineering feasibility. Among the more important factors are that any site should be well away from areas that are seismically or volcanically active and should avoid areas of high relief where seafloor sediments are unstable. It is also important to show that the area has been geologically stable over a time greater than the timescale of waste decay. The disposal medium should have low water permeability and low coefficients of ionic diffusivity. For on-the-seabed disposal, physical oceanographic considerations are judged most important, but in our present state of knowledge it is much more difficult to determine what, if any, site-specific considerations should apply in this case.