The most controversial and significant aspect of U.S. immigration policy concerns those persons who enter the country illegally in order to seek employment. It is known that a significant proportion of the "temporary" immigrants remain--authorities estimate that between three and six million undocumented aliens live permanently in the U.S., a figure that grows by the hundreds of thousands each year--but other aspects of the issue are less clear. There is no consensus about how the importation of foreign workers affects the U.S. labor market, nor about the desirability of some system to...