This report describes the performance of certain types of engineered barriers, e.g. backfills for excavated areas, buffers between the waste container and the host rock, and seals for boreholes, shafts and drifts, in a deep geological repository system for the disposal of high level radioactive wastes. In addition, it provides technical information relevant to their development. Contents: 1. Introduction; 2. Objectives and scope; 3. Performance of the backfill and sealing systems and their evaluation; 4. Repository design concepts; 5. Seal performance functions; 6. Laboratory and field testing; 7. Summary and conclusions.