This books aims to provide an introduction to various techniques to determine the pressure and temperature conditions of formation of metamorphic rocks. The necessary thermodynamic foundations and principles, and the basis of geothermobarometric methods, are carefully derived. Special emphasis is placed on the use of phase diagrams to determine the conditons of formation and to unravel the PT paths of metamorphic rocks during orogeny. The book is divided into three parts. Part A introduces some of the broader aspects of mineral solid solutions, Part B discusses the theoretical basis of geothermometry and geobarometry, and Part C deals with phase diagrams. Many examples are incorporated into the main body of the text to enable the reader to "learn-by-doing".