When more than twenty million immigrants arrived in the United States between 1880 and 1920, the government attempted to classify them according to prevailing ideas about race and nationality. But this proved hard to do. Ideas about racial or national difference were slippery, contested, and yet consequential—were "Hebrews" a "race," a "religion," or a "people"? As Joel Perlmann shows, a self-appointed pair of officials created the government's 1897 List of Races and Peoples, which shaped exclusionary immigration laws, the wording of the U.S. Census, and federal studies that informed social policy. Its categories served to maintain old divisions and establish new ones. Across the five decades ending in the 1920s, American immigration policy built increasingly upon the belief that some groups of immigrants were desirable, others not. Perlmanntraces how the debates over this policy institutionalized race distinctions—between whites and nonwhites, but also among whites—in immigration laws that lasted four decades. Despite a gradual shift among social scientists from "race" to "ethnic group" after the 1920s, the diffusion of this key concept among government officials and the public remained limited until the end of the 1960s. Taking up dramatic changes to racial and ethnic classification since then, America Classifies the Immigrants concentrates on three crucial reforms to the American Census: the introduction of Hispanic origin and ancestry (1980), the recognition of mixed racial origins (2000), and a rethinking of the connections between race and ethnic group (proposed for 2020).