This dissertation project examines the development of Argentine criminology in the years 1890-1930. It explores how this new discipline, which at the turn of the century included legal medicine, psychiatry, neurology, anthropology, and sociology, played a critical role in political debates on immigration, class formation, and national identity. It argues that these criminologists helped to delineate the boundaries of citizenship and nationhood in a rapidly modernizing society rife with true and imagined social problems. Argentine criminologists, many of whom were trained in Europe, sought to become vital players in the international scientific community. At the same time, they were engaged in a conscious project to build a distinctly Argentine science geared to solving Argentine social problems. The project documents how the Argentine political and intellectual elite used the language and scientific concepts of criminology to articulate their fears of social unrest, political chaos, and property loss. Moreover, it demonstrates how, as criminologists sketched new definitions of the criminal and proposed new criminal types, they as importantly constructed new ideals of the good Argentine citizen. The project first traces the intellectual formation of criminology as a discipline, its institutionalization, and its rise to international status. Second, the project describes the impact and realization of new criminological ideas in government policies, specifically in policy techniques, courtroom procedure, and legislative decisions. Finally, it concludes with a discussion of the impact of criminological concepts on the increasingly narrow requirements for Argentine citizenship in the first decades of the twentieth century.