A proper understanding of the causes of international migration suggests that punitive immigration and border policies tend to backfire, and this is precisely what has happened in the case of the United States and Mexico. Rather than raising the odds that undocumented immigrants will be apprehended, U.S. border-enforcement policies have reduced the apprehension rate to historical lows and in the process helped transform Mexican immigration from a regional to a national phenomenon. The solution to the problems associated with undocumented migration is not open borders, but frontiers that are reasonably regulated on a binational basis.