Santa Cruz Province, Argentina, has some of the richest fossil mammal localities in the world. However, the absolute and relative ages of its fossil localities have long been a source of confusion and debate. In particular, there has been longstanding disagreement about the relative ages of the fossils from the western part of the province in deposits of the Pinturas Formation compared with those from the numerous localities of the Santa Cruz Formation along the Atlantic coast. Drawing on recent studies of the tuffaceous sediments in many classic fossil localities, and studies of fossil representatives of marsupials, rodents, and primates, we provide a synthesis of the temporal relationship among fossil localities throughout the province. There is broad agreement between the results of the tephrochronology and mammalian paleontology. Both tephra correlations and paleontological comparisons indicate that the lower units of the Pinturas Formation are older than the sections of the Santa Cruz Formation preserved at Monte León and Cerro Observatorio, supporting Ameghino's suggestion that part of the Pinturas Formation represents a distinct faunal zone. However, the upper unit of the Pinturas Formation seems to correspond in age with the lower part of the sections at Monte León and Cerro Observatorio.