For much of the twentieth century, Immigration and Naturalization Service (INS) officials recognized that the US-Mexico border region was different. Here, they confronted 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 creating the nation's immigration laws and policies for the borderlands. In The INS on the Line, S. Deborah Kang 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. Through a nuanced examination of the agency's legal innovations in the Southwest, Kang demonstrates that the agency defined itself not only as a law enforcement unit but also as a lawmaking body. In this role, the INS responded to the interests of local residents, businesses, politicians, and social organizations on both sides of the US-Mexico border as well as policymakers in Washington, DC. Given the sheer variety of local and federal demands, 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. The composite approach to border control developed by the INS continues to inform the daily operations of the nation's immigration agencies, American immigration law and policy, and conceptions of the US-Mexico border today.