As of Jan. 23, all U.S. citizens must have a passport for airline travel to Mexico and the Caribbean. But, those traveling by land or boat need only a driver's license or birth certificate. (The U.S. government may require passports for land travel as early as Jan. 1, 2008, although it is still debating an extension.) read more
You need a U.S. passport to cross the border into Mexico in most cases. You can use a passport card if you're driving, but need a passport book for air travel. read more