For high level and alpha bearing radioactive wastes, the disposal concept most generally agreed upon is the emplacement of the conditioned waste in deep underground respositories in stable geological formations. The report reflects the general consensus that long lived radioactive wastes can be isolated safely from human environment in such repositories located in deep, geological formations. One of the most important tasks in the development of underground disposal systems is the plugging of boreholes and the sealing of shafts excavated for either exploratory or operational purposes. Several countries have outlined proposals and identified geological formations for detailed examination as potential host media for waste repositories. Contents: 1. Introduction and scope; 2. Background information; 3. Performance specifications; 4. Materials and methods; 5. Longevity; 6. Design factors; 7. Design of sealing systems; 8. Performance testing; References; Glossary.