Processes of migmatization provide a challenging field for petrological interpretation. Yet the study of migmatites has been fraught with conflicting theories for many years. Against this background the authors review the important aspects of the petrology of migmatites. In accordance with current thinking, they concentrate on partial melting and its consequences, but also consider other possible origins. Recent progress in understanding migmatization has largely arisen from studies of simple cases. This knowledge has been applied by several of the authors to complex polymigmatitic terrains and migmatite petrogenesis.