The broad outlines of the structure and metamorphic evolution of southern Brittany, France, are outlined. There were two metamorphic cycles, Precambrian and Hercynian, each subdivided into a preorogenic phase of ectinite formation and a synorogenic phase of metasomatic granitization. Differences in the form and structure of the Hercynian massifs represent differences in tectonic conditions at the time of formation. All the granites belong to the same family, from the anatectic granite to the surface microgranites. Three stages can be distinguished in the tectonic history of the Hercynian range on the basis of genetic relationships of the granites.