This paper summarizes the principles of chemical thermodynamics in a form which is convenient for dealing with problems encountered in metamorphic petrology. Particular effort has been made to illustrate thermodynamic concepts by means of graphical representation. The concept of the equilibrium constant and the choice of standard states are discussed in detail. A brief introduction to non-ideal solutions deals with relatively simple binary solution models based on the method of Margules. Various applications of thermodynamics and guides for specific calculations are presented, with some emphasis on oxidation and sulphidation reactions.