Categories of race (ethnicity, color, or both) have appeared and continue to appear in the demographic censuses of numerous countries, including the United States and Brazil. Until recently, such categorization had largely escaped critical scrutiny, being viewed and treated as a technical procedure requiring little conceptual clarity or historical explanation. Recent political developments and methodological changes, in US censuses especially, have engendered a critical reexamination of both the comparative and the historical dimensions of categorization. The author presents a comparative analysis of the histories of racial/color categorization in American and Brazilian censuses and shows that racial (and color) categories have appeared in these censuses because of shifting ideas about race and the enduring power of these ideas as organizers of political, economic, and social life in both countries. These categories have not appeared simply as demographic markers. The author demonstrates that censuses are instruments at a state's disposal and are not simply detached registers of population and performance.