This accessible text is an introduction to the theory of phase transitions and its application to real materials. Assuming some familiarity with thermodynamics and statistical mechanics, the book begins with a primer on the thermodynamics of equilibrium phase transitions, including the mean-field and Ginzburg-Landau approaches. The general kinetic features and dynamics of phase transitions are explained, ensuring that readers are familiar with the key physical concepts. With the foundations established, the general theory is applied to the study of phase transitions in a wide range of materials including ferroic materials, caloric materials, liquid crystals and glasses. Non-equilibrium phase transitions, superconductors and quantum phase transitions are also covered. Including exercises throughout and solutions available online, this text is suitable for graduate courses as well as researchers in physics and materials science seeking a primer on popular and emerging research topics.