Evidence from eighteenth-century marriage applications in Mexico City and Cadiz reveals that migration from Spain to the New World was primarily an extension of domestic movements from rural to urban areas, not the direct result of transatlantic networks. The migratory dynamism that pervaded Spanish society fueled Spain’s fledgling urbanization in the era of commercial capitalism, as peasants increasingly moved to towns and cities, especially to Cadiz. Many of these internal migrants subsequently used the social capital and other resources that they had accumulated in Cadiz and elsewhere on the Iberian Peninsula to facilitate migration to the New World.