Patterns of state-building in Latin America varied greatly during the 20th century, but the nature and origins of these differences are still poorly understood. This dissertation offers a novel explanation for the long-term institutional development of Latin American countries, based on the social origins of state-building. The argument traces these differences to a critical juncture that was initiated with changes in the world political economy after the outbreak of the first World War in 1914. These changes allowed local political actors throughout the region to develop a certain degree of autonomy from economic elites as well as foreign powers and initiate projects of state reform with the support of newly created social coalitions. I identify three paths of state building: a professionalizing route, which characterized the trajectories of Brazil and Chile, where reformers were able to garner the support of middle and upper-class actors to build a technically competent, but limited state-apparatus. In a second path, followed by Argentina and Mexico, reformers built a populist coalition of middle and lower-class actors aimed at attracting large segments of society into the realm of interactions with state institutions, enlarging their scope without significant professionalization. Finally, a third, gradualist route, was taken in Uruguay and Costa Rica, where reformers forged a very broad social coalition to incrementally professionalize and expand the reach of the state apparatus. These different routes of institutional building had effects not only on crucial state-capacity outcomes such as the ability to successfully tax and regulate society and the extent to which state authorities can garner compliance from the citizenry, but also on regime outcomes, determining the extent and degree to which members of the population of these countries became active members of their respective polities and whether or not they could effectively influence the levers of political power.