The first edition of this book was published in 1965 and its French translation in 1966. The revised second edition followed in 1967 and its Russian translation became available in 1969. Since then, many new petrographie observations and experimental data elucidating reactions in metamorphie rocks have made a new approach in the study of metamor phic transformation desirable and possible. It is feIt that this new approach, attempted in this book, leads to a better understanding of rock metamorphism. The concept of metamorphie facies and subfacies considers asso cations of mineral assemblages from diverse bulk compositions as char acteristie of a certain pressure-temperature range. As new petrographie observations accumulated, it became increasingly difficult to accommo date this information within a manageable framework of metamorphic facies and subfacies. Instead, it turned out that mineral assemblages due to reactions in common rocks of a particular composition provide suita ble indieators of metamorphie conditions. Metamorphic zones, defined on the basis of mineral reactions, very effectively display the evolution of metamorphic rocks. Thus, the importance of reactions in metamor phic rocks is emphasized. Experimental calibration of mineral reactions makes it possible to distinguish reactions which are of petrogenetic sig nificance from those which are not. This distinction provides guidance in petrographie investigations undertaken with the object of deducing the physieal conditions of metamorphism. Within a metamorphie terrain, points indicating the same reaction constitute a line or a band, here designated by the term isoreaction-grad.