This chapter describes the background and basis of ways of disposing high-level radioactive waste in the form of spent reactor fuel or vitrified waste in crystalline rock. Guidance in the precise location of the tunnels is provided by long pilot core drillings that give information on the presence of major fracture zones in the potential host rock. Backfilling of tunnels and shafts is made for two major purposes: reestablishment, as well as possible, of the original degree of tightness of the rock, which requires that the material brought into them has the same hydraulic conductivity as the rock that it replaces; and hindrance of unwanted intrusion of terrorists or people wanting to get access to radioactive materials. The chapter concludes by describing how part of mined repositories can be used as underground laboratories and what types of tests can be made there.