The application of stable isotope fractionation to geothermometric problems is introduced through considerations of statistical thermodynamics first developed by H. C. Urey. The available calibrations of isotope fractionation between species of geochemical interest are reviewed. These calibrations are discussed in the light of simple theoretical considerations, and a new set of thermometric equation is derived for the most common mineral constituents of rocks. Examples of application are given for igneous and metamorphic rocks. The significance of ‘isotopic’ temperatures is discussed in terms of environmental and kinetic considerations. As an example of possible future developments we derive the relationship between observed isotope temperatures and the speed of ascent of a rock system during a postorogenic isostatic readjustment.