U.S. Citizen Passport Requirement
The Western Hemisphere Travel Initiative (WHTI) once fully implemented will require all travelers entering or reentering the United States to be in the possession of a valid passport.
Anticipated Implementation
October 1, 2007 - The departments of State and Homeland Security reestablished the valid passport requirements for air travel. Valid passports are required for air travel to / from Canada, Mexico, the Caribbean, and Bermuda.
U.S. lawful permanent residents will continue to be able to use their Alien Registration Card (Form I-551) issued by the Department of Homeland Security or other valid evidence of permanent residence status to apply for entry to the United States.
As early as Summer 2008 - A valid passport will be required for all sea and air travel. And subject to U.S. Government amendment, U.S. and Canadian citizens 15 or younger with their parents consent may cross the U.S./Canadian border by land or sea with a certified copy of their birth certificate.
Until that time
Sea Travel Only For domestic travel which includes: the Bahamas, Bermuda, Canada, the Caribbean and Mexico, a passport (valid or expired) is highly recommended.
In the absence of a passport, a birth certificate (original or certified copy), plus laminated picture ID card issued by a federal, state, or local government agency is required. Note: Baptismal papers and hospital certificates of birth (except for new borns) are not acceptable.
For U.S. Naturalized citizens, in the absence of a passport, Naturalization papers (either original or notarized copy) plus a picture ID card issued by a federal, state, or local government agency is required.
A voter registration card or Social Security Card are not considered to be proof of citizenship.
Children under 16 years of age do not require a picture ID.
International Travel A valid passport is required; visas are required where they apply. This includes Europe, Asia, Central and South America.