This chapter deals with changes in constitution on the microscale and macroscales of smectite clays by processes in nature or by anthropological impact. It deals in particular with the longevity of smectite clays in bentonite beds and in the form of canister-embedding buffers in repositories for deep geological high-level radioactive waste (HLW) disposal. The basis is the concept of fundamental clay crystal units described in Chapter 3, the clay-fluid interaction touched upon in Chapter 4, and the microstructural models discussed in Chapter 5. Most of the message is inherited from decades of research by my own staff but a number of new findings of international experts from Denmark, Germany, and Vietnam have added to the present knowledge.