There are few more sprawling and unruly areas of academic inquiry than the law of immigration policy in the United States and around the world. Three reasons readily come to mind for explaining the inherent difficulties in this area.

The first is the sprawling nature of the problem. Immigration into any country comes in a large number of different forms. Unlawful immigrants cross national borders to take low-paying jobs that are still better than those available to them back home. Highly trained workers in great demand are enticed by countries because they possess specialized skills, often in very technical disciplines, that are not easily filled by domestic workers. Family members living across borders often want to reunite, and those who are living in the same country often want to remain together, even though frequently some family members do not have permission to remain. Refugees fleeing repression and civil war seek asylum across borders. States must decide what to do about each of these complex situations. The diversity and intricacy of the problems helps explain why there is so little consensus in the United States—or in nearly any other advanced western democracy for that matter—about who should get to enter, and on what terms.

The second is that there is no consensus as to the general approach to immigration issues. For starters, there is no serious policy analyst who thinks that an open immigration policy is workable in modern times. Whatever the situation a century ago, under that open immigration policy today, tens of millions of persons could move to the Unites States simply by walking across a border checkpoint or buying a one-way ticket to any one of dozens of points of entry into the United States. These migrants would put tremendous pressure on the public goods provided by the modern welfare state. Even were they denied citizenship, the burdens on the educational and healthcare system would be great. And were they granted citizenship as of right, it would dramatically reshape the American polity in unpredictable ways. Consequently, neither equilibrium seems stable.

As radical as open borders would be, certainly today no one is in favor of the opposite extreme—a per se ban on immigration that could easily result in vast dislocations of its own by depriving the United States of much-needed specialized labor, keeping families apart, and so forth. Yet ruling out the extremes of the policy space does little to tell us which of the many different positions on immigration and naturalization should be adopted. So the purpose of all the articles within this Issue is to search for some viable middle ground. That search in turn raises serious questions of institutional design.

This leads to the third challenge. In all legal settings, policy makers are forced to choose between a set of ex ante restrictions that they apply in order to forestall harm and a set of ex post sanctions against antisocial behavior. Immigration law is no different. At a basic level, the challenge of system design is to decide whether to concentrate ex ante on the entry of immigrants into the United States or instead to try, ex post, to control and sort among immigrants after they have arrived. The first approach asks how to structure border control and the application process for new arrivals in order to pick desirable migrants. The second approach asks how to identify these migrants (as well as encourage desirable behavior) after entry, using some combination of civil and criminal sanctions, as well as the threat of apprehension, detention, and ultimately deportation. It is well understood that each of these systems of social control are likely to have unforeseen and complex interactions with the other systems that are in place, which in turn requires delicate judgments on how best to run the immigration system from a holistic and integrated perspective.

The articles that are found in this Issue all address at least one of these issues. What follows are brief summaries, in alphabetical order, of the articles included in this Issue. Taken together, the articles themselves are ample evidence of the durable intractability of the underlying issues.