Mineral assemblages of igneous and metamorphic rocks may reflect the approach of a rock to the equilibrium during its formation. The accurate measurements of the thermodynamic properties of minerals, melts and volatile fluid phases, combined with experimental and theoretical studies of their mixing properties, have made it possible to calculate the equilibrium conditions for particular rock systems. The equilibrium thermodynamics provides a way of looking at rocks, not only for discovering at what conditions they formed, for example the temperature and pressure of formation, but also for understanding the processes involved in their formation. This book provides three examples to use the equilibrium thermodynamic in geological problems in different scale. First chapter deals with orogenic scale exhumation mechanism of the high-grade metamorphic rocks. Second chapter describes sources of heating and fluid composition change during metamorphism of upper crustal rocks. While the millimeter scale geological problems are discussed in chapter three which deals with the formation mechanisms of the strain cap minerals. Geoscientists and chemists will find this book essential reading.