Abstract: This paper empirically documents the continued importance of the legal families (common law and civil law) for the diffusion of formal legal materials from the core to the periphery, and some possible channels of diffusion, in post-colonial times. This raises the possibility that substantive differences between countries of different families around the world, such as those documented in the legal origins literature, continue to be the result of separate diffusion processes rather than of intrinsic differences between common and civil law. Using the example of corporate and securities law, this paper documents the frequent and often exclusive use of legal materials and models from the respective legal family’s core countries in treatises and law reform projects in thirty-two peripheral and semi-peripheral countries. Most authors of these treatises and projects were trained in the respective core countries. Data on the activities of national legal development and cooperation organizations, trade and investment flows, and student migration confirm the close legal family ties and provide some evidence of possible channels through which materials may continue to diffuse within their legal families after decolonization. The diffusion of formal legal materials need not imply that the substantive development of law is affected by foreign influences, at least not in ways that induce substantive differences between periphery countries of different legal families. Various theories from comparative law, sociology, political science, and economics provide reasons, however, why the content of law in the periphery might continue to be influenced by core country models of the same legal family, as the evidence of formal diffusion suggests they are. Such diffusion theories fit the available data better than other theories put forward in the literature.