For much of the twentieth century, Immigration and Naturalization Service (INS) officials recognized that the US-Mexico border region was different, confronting a set of political, social, and environmental obstacles that prevented them from replicating their achievements on Angel Island and Ellis Island, the most restrictive immigration stations in the nation. In response to these challenges local INS officials resorted to the law, nullifying, modifying, and even inventing immigration laws and policies for the borderlands. The INS on the Line traces the ways in which the INS on the US-Mexico border made and remade the nation's immigration laws over the course of the twentieth century. While popular and scholarly accounts describe the INS primarily as a law enforcement agency, the author demonstrates that the agency defined itself not only as a law enforcement unit but also as a lawmaking body. Through a nuanced examination of the agencys legal innovations in the Southwest, the author reveals how local immigration officials constructed a complex approach to border control, an approach that closed the line in the name of nativism and national security; opened it for the benefit of transnational economic and social concerns; and redefined it as a vast legal jurisdiction for the policing of undocumented immigrants. Despite its contingent and local origins, this composite approach to border control continues to inform the daily operations of the nation's immigration agencies, American immigration law and policy, and our very conceptions of the US-Mexico border today.