As Congress debates the shape of a new immigration act for the United States, one issue crucial to the debate has been surprisingly neglected: Whatever Congress and the administration decide is an appropriate immigration policy in the 1980s and beyond, can these decisions be enforced? Is it possible for the United States to implement and enforce any immigration policy, regardless of its content? We must raise this question because there is good evidence that our ability to enforce immigration policies has broken down. A narrow but crucial issue must be examined: Can the United States control the flow of immigration?