How does emigration of highly educated citizens of low-income countries to high-income countries affect the economies of the origin countries? The direct effect is “brain drain”—a decrease in the country’s human capital stock. However, there may also be indirect “brain gain” effects. This review summarizes evidence that uses causal inference methods to reveal mechanisms that may lead to brain drain, gain, or circulation. Collectively, the weight of the evidence suggests that migration opportunities often increase human capital stock in origin countries and produce downstream beneficial effects through remittances; foreign direct investment and trade linkages; transfers of knowledge, technology and norms; and return migration. We discuss conditions under which benefits from skilled migration may outweigh costs and also describe potential research paths to inform policy.