Urbanization is an important demographic phenomenon, and in Latin America it transformed the settlement pattern from rural to predominantly urban in less than 50 years. Understanding the biological consequences of this change in settlement pattern is an important challenge for human biologists. One approach to understanding the effects of urban environments on human biology has been to study rural-to-urban migrants. In Latin America this research has shown that 1) the fertility of migrants tends to be intermediate between that of rural and urban populations, and 2) migrants tend to suffer higher rates of mortality and morbidity, at least initially, than long-term urban residents. There is some indication that the actual physical conditions under which migrants live in urban areas—and these tend to be among the most impoverished—are more important variables than migrant status per se. Studying rural-to-urban migrants requires careful attention to a number of conceptual issues. One issue is the definition of rural and urban. These two types of settlements are no longer as distinct as they once were, and “urban” can mean very different things in different places. Another issue is the complexity of current migration patterns. The classic case of people moving from a distinctly rural setting to a distinctly urban one and staying there for the remainder of their lives is not the norm. Third, the urban environments of large cities are extraordinarily heterogeneous environments with enormous socioeconomic differentials in health. Hence, it matters where in the urban environment the migrants live. Am. J. Hum. Biol. 16:395–404, 2004. © 2004 Wiley-Liss, Inc.