Traditionally, U.S. citizens, Canadian citizens and Mexican citizens with Border Crossing Cards have been able to enter the United States from points in the Western Hemisphere without passports. However, starting January 23, 2007, all persons entering the United States by air from other Western Hemisphere countries will be required to have a passport. This includes U.S. citizens returning to the U.S. U.S. citizens who travel to Canada, Mexico or the Caribbean by air will be required to have a passport to return to the United States.

The new requirement is part of the U.S. government's Western Hemisphere Travel Initiative which is required by legislation enacted in 2004. The measure is intended to improve security by no longer allowing persons, including U.S. citizens, to enter the United States using a birth certificate and driver's license. The requirement applies initially only to persons traveling by air. Starting in 2008, persons entering the United States by land or sea will also be required to carry a passport or other acceptable document. The U.S. government is considering which documents will be acceptable alternatives to a passport.