Many materials used in industry are crystals. These crystals often show anomalies, such as sudden softening or embrittlement at certain temperatures. If controlled, such behavior 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 behavior, and provides an insight into the important thermodynamic principles and microstructural properties involved. It starts with the fundamental principles of structural phase transitions in materials. Ferroelasticity, twinning and related microstructures are then 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.