The foreign-born population of the United States today comprises approximately 13.5 percent of the total, still below the peak of 14.8 percent in 1890. But numbers are not the real heart of contemporary debates. The question underlying all these works, and much political discourse, is whom the nation welcomes. Nativism and racism explain Americans’ desire to restrict immigration, and tend to stay fairly consistent over time. How those restrictions have been envisioned and implemented has changed. In 1924 the US not only restricted the absolute number of immigrants who could come, it denied entry to immigrants from half the world on explicitly racial grounds, and among those permitted entry, it drew explicit distinctions between those considered desirable and undesirable. These restrictions and the measurement of national origins engrained in them amounted to the social engineering of the future racial make-up of the American people. The racial logic of the restriction era remains with us. The proposed border wall, whether structural or symbolic, in many ways echoes its precedents. Restrictions on immigration and citizenship have always functioned as legal walls, drawing imaginary lines around the nation and its people, to the exclusion of many both within and without.