We bring to bear a novel dataset covering the employment history of about 450 million individuals from 180 countries to study return migration and the impact of skilled international migration on human capital stocks across countries. Return migration is a common phenomenon, with 38\% of skilled migrants returning to their origin countries within 10 years. Return migration is significantly correlated with industry growth in the origin and destination countries, and is asymmetrically exposed to negative firm employment growth. Using an AKM-style model, we identify worker and country-firm fixed effects, as well as the returns to experience and education by location and current workplace. For workers in emerging economies, the returns to a year of experience in the United States are 59-204% higher than a year of experience in the origin country. Migrants to advanced economies are positively selected on ability relative to stayers, while within this migrant population, returnees exhibit lower ability. Simulations suggest that eliminating skilled international migration would have highly heterogeneous effects across countries, adjusting total (average) human capital stocks within a range of -60% to 40% (-3% to 4%).