The tectonic evolution of SW Dyfed is presented in terms of a model of basin evolution and subsequent destruction by reactivation of the basin bounding faults in contractional mode. Movements along long-lived Palaeozoic extensional faults were reversed during Variscan compression, with the pre-existing faults accommodating the contraction in addition to the development of a thin-skinned thrust and fold system. Basin analysis of the pre-Variscan extensional system uses sedimentological and stratigraphical evidence to constrain the timing of movements of the basin bounding faults, thus allowing division of the stratigraphy into depositional megasequences related to these extensional movements. Field examples on all scales illustrate active extension during the Palaeozoic. Structural cross sections through SW Dyfed demonstrate the interrelationship of the extensional and contractional deformations, with palinspastic restorations of the cross sections illustrating the complexity of the pre-Variscan template.