Published by the American Geophysical Union as part of the Short Courses in Geology Series, Volume 7.A renaissance in the study of metamorphic rocks has occurred in the past decade. For a long time (with some notable exceptions) metamorphic rocks were studied as examples of heterogeneous systems at equilibrium and the study of metamorphism was, in effect, a phase equilibria experiment. A major goal of metamorphic petrology was to pigeonhole the types of mineral assemblages encountered into the appropriate metamorphic facies. Indeed, even geothemobarometry, in all its sophistication, is nothing but a quantitative form of pigeonholing. However, all of this has changed recently and today, metamorphic rocks are studied in large part as a means to unravel major tectonic problems during orogenesis.