(From CBP)
ALL persons* traveling by air between the United States and Canada, Mexico, Central and South America, the Caribbean, and Bermuda are required to present a valid passport or Air NEXUS card.

Oral declarations are no longer accepted from U.S., Canadian or Bermudian travelers seeking to enter the U.S. by sea and/or land.

June 1, 2009 – ALL persons* (LPR exceptions below) traveling between the U.S. and Canada, Mexico, Central and South America, the Caribbean, and Bermuda by land or sea (including ferries), will be required to present a valid passport, U.S. passport card, trusted traveler program card (NEXUS, SENTRI or FAST card ) or an Enhanced Driver’s License currently issued only in Washington State, Michigan, New York, and Vermont.

*Lawful permanent residents (LPRs), refugees, and asylees (of the United States) will continue to be able to use their Alien Registration Card (Form I-551), issued by the Department of Homeland Security (DHS), or the Travel Document issued to those with refugee or asylee status to apply for entry to the United States.  LPR’s may still need to present a passport for entry into a foreign destination.  For this reason airlines may deny boarding to LPR’s traveling without a passport.  NOTE:  You may want to contact the airlines prior to traveling to confirm their documentation requirements.
A foreign national or alien entering the U.S. is generally required to present a passport and valid visa issued by a U.S. Consular Official, unless they are a citizen of a country eligible for the Visa Waiver Program, or are a lawful permanent resident of the U.S. or a citizen of Canada. The Visa Waiver Program allows foreign nationals from certain countries to be admitted to the U.S. under limited conditions and for a limited time without obtaining a visa. The foreign national must arrive on an approved carrier (if coming by air or sea), staying no more than 90 days, for pleasure/medical purposes, and be able to prove they are not inadmissible. The foreign national is still required to have a passport. To obtain a list of countries eligible for the Visa Waiver Program, please reference the Department of State Web site.

Foreign nationals that are lawful permanent residents of the U.S. must present a Permanent Resident Card (“Green Card”, INS Form I-551), a Reentry Permit (if gone for more than 1 year), or a Returning Resident Visa (if gone for 2 years or more) to reenter the United States. U.S. LPRs are not required by the U.S. to have a passport for travel within the Western Hemisphere, although most foreign destinations will require you to have one to enter their country. (LPRs that are out of the U.S. for more than 180 days are subject to new immigrant inspection procedures as per 8 USC 1101.)

Foreign nationals that have applied for permanent residency or a change of status may need to be approved for advance parole in order to be readmitted to the U.S. after traveling abroad.

Canadians coming as a Treaty Trader, classification E are required to have a visa to enter the U.S.

A visa and passport are not required of a Mexican national who is in possession of a Form DSP-150, B-1/B-2 Visa and Border Crossing Card, containing a machine-readable biometric identifier, issued by the Department of State and is applying for admission as a temporary visitor for business or pleasure from contiguous territory by land or sea.

Continuing students who are going to travel outside of the United States must see their foreign student advisor and obtain an endorsement from the DSO or RO. The endorsement will be made on page 3 of the SEVIS Form I-20 or page 1 of the DS-2019. When returning to the United States, a continuing student/exchange visitor must present a valid SEVIS Form I-20 or DS-2019 with the DSO or RO signature showing that the student is active and in good standing with the school or program.

If you are traveling with children, you should also see our Q&A titled: Traveling with children, documents, paperwork, and identification. For more information on how to obtain a nonimmigrant visa, contact the Department of State.  Be aware, the Department of State can authorize travel to the U.S., however, a visa does not guarantee that you will be allowed to enter the U.S. CBP has the authority to grant or deny your admission to the United States. U.S. Citizenship and Immigration Services determine how long you may stay in the United States and whether or not you may extend your stay. You may contact them at 1-800-375-5283.