Any individual who is a citizen of the United States must provide an accepted form of documentation that verifies his or her identity. The most common and widely accepted document is a valid (unexpired) U.S. passport. Members of the U.S. armed forces--including the Army, Navy, Air Force, Marines, or Coast Guard --may present their military IDs and travel orders in lieu of a passport. Citizens enrolled in the NEXUS Program, a U.S. and Canadian initiative created to expedite travel between the two countries, may use their NEXUS cards at participating airports and other points of entry.
A "permanent resident" is any foreign-born individual who is legally permitted to live and work in the U.S. All U.S. residents are required to carry a Permanent Resident Card, also known as a "green card," to identify their status. Residents who re-enter the U.S. by land or sea must present their green card at the border. Air travelers, however, must also possess a valid passport or other secure travel document from their country of origin.
Foreign travelers who are not U.S. permanent residents must present a valid passport issued by their country of origin. However, citizens of Canada and Bermuda who travel by land or sea may provide other documents approved under the Western Hemisphere Travel Initiative (WHTI), such as a NEXUS card or enhanced driver's license. While Mexico is also a participant in the Western WHTI, Mexican citizens are not exempt from the passport requirement.
