Polycyclic gneiss complexes in which early rock-assemblages of granulite or high amphibolite facies have been tectonically and metamorphically reworked occur on a regional scale in the crust. The distinctive properties of the dry, structurally isotropic parent-assemblages have played an important part in determining the style of regeneration which was characteristically inhomogeneous. The reintroduction of water and consequent amphibolization of granulites appears to have been associated with increases of ductility and consequent changes in tectonic behaviour. During the early stages of Proterozoic mobility the Archaean crust was traversed by incipient dislocations which developed into `straight belts' characterized by high strain. Deformation in the mobile provinces as a whole was associated with relative movements of the blocks defined by the straight belts. In these intervening blocks, regeneration was most effective at intermediate depths where pore fluids were available. Many Archaean granulites lying beneath the domain of regeneration remained as closed systems and suffered little tectonic or metamorphic modification.