Between 1850 and 1930, demographic upheaval in the United States was connected to reorganization of the racial order. Socially and politically recognized boundaries between groups shifted, new groups emerged, others disappeared, and notions of who belonged in which category changed. All recognized racial groups-blacks, whites, Indians, Asians, Mexicans and others-were affected. This article investigates how and why census racial classification policies changed during this period, only to stabilize abruptly before World War II. In the context of demographic transformations and their political consequences, we find that census policy in any given year was driven by a combination of scientific, political, and ideological motivations.Based on this analysis, we rethink existing theoretical approaches to censuses and racial classification, arguing that a nation's census is deeply implicated in and helps to construct its social and political order. Censuses provide the concepts, taxonomy, and substantive information by which a nation understands its component parts as well as the contours of the whole; censuses both create the image and provide the mirror of that image for a nation's self-reflection. We conclude by outlining the meaning of this period in American history for current and future debates over race and classification. Between the Civil War and World War II, the United States underwent a profound process of racial reorganization. Officially recognized group categories expanded and contracted; socially recognized boundaries between groups blurred and shifted; citizens and public actors passionately debated who belonged in which group. Basic components of the racial order were revised, revisited, and fundamentally altered. Such debates were highly consequential. While whites never lost their position at the top of the status hierarchy, who belonged in this privileged group was hotly contested. Whether or not a given group or individual was included in the category of "white" profoundly affected that group's or person's social standing. Blacks and Chinese were placed into Our thanks to Traci Burch and Vesla Weaver for their help; this article grows out of a book project co-authored by them and Jennifer Hochschild. Thanks also to K. Miya Woolfalk and Ariel Huerta for their excellent research assistance, to Rodney Ross for assistance at the National Archives, and to