As a preferred barrier for high-level radioactive waste repositories, clay rock undergoes complex thermo-hydro-mechanical (THM) processes under decay heat. This introductory chapter reviews key aspects of this behavior. It summarizes temperature effects on the strength, deformation, and creep of clay rock, alongside the hydraulic properties of both intact and fractured material. The underlying physical mechanisms driving these thermal effects are discussed. Furthermore, the chapter surveys critical constitutive models for simulating THM coupling, including thermo-mechanical, thermo-creep, and thermal-hydraulic models. This review sets the foundation for the book’s subsequent in-depth analytical and numerical studies on THM behavior in geological disposal systems.