The last fifteen years have witnessed an amazing development of petrology. During this time it became readily feasible to investigate reactions at high temperatures and pressures. The new experimental techniques were immediately applied in the fields of mineralogy and petrology and, at present, research activity continues unabated. The aim of these in­ vestigations is the elucidation of the origin of magmatic and, particularly, of metamorphic rocks. Only a few years ago, the second editions of the well-known textbooks by TURNER and VERHOOGEN (1960) and by BARTH (1962) were published. But even since that time, our knowledge of metamorphic petrology has been augmented by numerous experimental investigations and by new petrographic observa­ tions as well. Such rapid growth warrents an evaluation of the accumulated knowledge bearing on the origin of meta­ morphic rocks. With this thought in mind, the present book was written. The treatment purposely stresses the mineral­ ogical-chemical aspects of metamorphism. The discussion is mainly concerned with the reactions, which transform the mineralogical composition of a rock, when subjected to meta­ morphic conditions within the earth's crust. "The question of the general relationship between the minerals and the mineral associations, on the one hand, and temperature and pressure, on the other, is the real core of the study of metamorphic rocks" (BARTH, 1962). Petrofabric analysis of metamorphic rocks is not discussed, because this is a special field of study.