Social scientists have sketched four distinct theories to explain a phenomenon that appears to have ramped up in recent years, the diffusion of policies across countries. Constructivists trace policy norms to expert epistemic communities and international organizations, who define economic progress and human rights. Coercion theorists point to powerful nation-states, and international financial institutions, that threaten sanctions or promise aid in return for fiscal conservatism, free trade, etc. Competition theorists argue that countries compete to attract investment and to sell exports by lowering the cost of doing business, reducing constraints on investment, or reducing tariff barriers in the hope of reciprocity. Learning theorists suggest that countries learn from their own experiences and, as well, from the policy experiments of their peers. We review the large body of research from sociologists and political scientists, as well as the growing body of work from economists and psychologists, pointing to the diverse mechanisms that are theorized and to promising avenues for distinguishing among causal mechanisms.