This chapter deals with the function of smectitic clay seals for isolation of waste and redirecting groundwater flow. The basis for this is modeling and calculation of the performance using laboratory-determined material properties and data from field testing, mainly concerning THMCB. For indicating the procedure in designing a repository and selecting functional parameters for comparing different concepts, we will focus on two of considerable potential use. One of them is very deep hole (VDH), known from Chapters 5 and 6 but looked at here with particular respect to site selection, constructability, and function. The other, representing the poor man’s choice, sarcophagus (SARC), is new and described and assessed. It represents a potentially effective procedure in the planning of a highly radioactive waste (HLW) repository from scratch.