This book traces the origins of the "illegal alien" in American law and society, explaining why and how illegal migration became the central problem in U.S. immigration policy--a process that profoundly shaped ideas and practices about citizenship, race, and state authority in the twentieth century. Mae Ngai offers a close reading of the legal regime of restriction that commenced in the 1920s--its statutory architecture, judicial genealogies, administrative enforcement, differential treatment of European and non-European migrants, and long-term effects. She shows that immigration restriction, particularly national-origin and numerical quotas, remapped America both by creating new categories of racial difference and by emphasizing as never before the nation's contiguous land borders and their patrol.