Resolving the high-level waste disposal question is one of the most serious problemas facing the nuclear industry today. To guarantee that nuclear waste never reaches the biosphere, researchers in several laboratories are devising a multiple-barriers concept, to prevent groundwater and corrosion from disturbing the waste disposal site. First, the waste is fused with glass, ceramic or synthetic rock into large cylindrical blocks sealed in stainless-stell canisters, which can be embodied in concrete blocks. Then the blocks are to be isolated for at least 10.000 years. To isolate the blocks by storing them in geologic formations seems to be the best compromise between safety and feasibility, as this is the only technology which will be available for the next several decades. The technology is available to drill holes, construct cavities, evaluate geological and hydroological conditions, etc., but there are still many questions to be answered, such as the very long-term stability of waste forms and its packaging under radiation, interations between waste and the geologic formation, and the long-term effects of heat flux on the geologic structures. Therefore, it is imperative that the necessary investigations move ahead, to select potential disposal sites in geologic formations, in order to start demonstration projects at these sites without further delay, because today there are allready thounsand of tons of spent fuel from commercial reactors in several countries waiting for safe disposal. To design an underground disposal facility, it is necessary to evalute the impact of the heat generated by the high-level waste on the surrounding rock. The last section of this monograph presents a methodology for determining the temperature field arround such a facility.