Although on a lesser scale than the United States, southern South America became a major receiving region during the period of mass transatlantic migration in the late 19th and early 20th centuries. Even as the white elites of most Latin American countries favored European immigration in the late 19th century, since in their eyes it would “civilize” their countries, it was the temperate areas closely tied into the Atlantic economy as exporters of primary products that received the bulk of European laborers. Previously scarcely populated lands like Argentina, Uruguay, and southern Brazil thus witnessed massive population growth and in some ways turned into societies resembling those of other immigration countries, such as the United States and Canada. This article concentrates on lands where the overwhelming majority of migrants headed, although it also briefly deals with Latin American nations that received significantly fewer newcomers, such as Mexico. This mass migration lastingly modified identity narratives within Latin America. First, as the majority of Europeans headed to sparsely populated former colonial peripheries that promised economic betterment, migration shifted prevalent notions about the region’s racial composition. The former colonial heartlands of Mexico, Peru, and northeastern Brazil were increasingly regarded as nonwhite, poor, and “backward,” whereas coastal Argentina, São Paulo, and Costa Rica were associated with whiteness, wealth, and “progress.” Second, mass migration was capable of both solidifying and challenging notions of national identity. Rather than crossing over well-established and undisputed boundaries of national identities and territories, migration thus contributed decisively to making them.