This textbook describes the fundamental principles of structural phase transitions in materials in an easily understandable form, suitable for both undergraduate and graduate students. Ferroelasticity, twinning and related microstructures are described. Landau-type theories of phase transitions are introduced, together with details of elastic and specific heat anomalies, the formation of spontaneous strain, and the generation of solitary waves at temperatures close to the transition point. Many materials used in industry are crystals. These crystals often show anomalies such as sudden softening or embrittlement at certain temperatures; if controlled, such behaviour can be extremely useful for manufacturing and high technology applications. This is one of the first books to describe the recently determined physical origins of such behaviour, and provides an insight into the important thermodynamic principles and microstructural properties involved.