Phase transitions are driven by pressure as well as temperature, and the use of pressure to tune the electronic structures of materials can help further our understanding of materials properties. Chapter 8 begins with basic considerations of the thermodynamics of materials under pressure, and how phase diagrams are altered by temperature and pressure together. Volume changes can also be driven by temperature through thermal expansion, and the concept of “thermal pressure” from nonharmonic phonons is explained. Electronic energy is responsible for big contributions of +PV to the free energy, and this chapter describes how electron energies are altered by pressure. Cross-terms between temperature and pressure are discussed. The chapter ends with a discussion of kinetic processes under pressure, and the concept of an activation volume.