This chapter introduces key concepts that are developed in this textbook. It describes the concept of microstructure and other features of materials that undergo interesting changes with temperature or pressure. These changes are motivated by the thermodynamic free energy, but require a kinetic mechanism for atoms to move. Chemical unmixing and ordering on a crystal lattice are described, and the kinetics of diffusion by vacancies is explained. The free energy is used to explain melting. A summary of essential aspects of thermodynamics and kinetics is given at the end of the chapter, including basic ideas of statistical mechanics and the kinetic master equation.