This chapter describes the principles of isolating low-level radioactive waste (LLW) and intermediate-level radioactive waste (ILW) according to two main principles of location: on-ground and underground. The latter implies placement in rock, of which crystalline rock is the focus. Engineered barriers are indispensable, with smectitic clay a major seal component and concrete used for support and providing stable space for placing most types of LLW and ILW. The function of clay barriers with respect to hydraulic and gas conductivities, diffusive transport of solvates, erodibility, and expandability is described, including summarized techniques for determining them. The chapter ends with a description of the release and transport of radionuclides and the definition of safe zones in lee of repositories.