Up until June 2009, you could re-enter the United States via land or sea simply by presenting a government-issued photo ID and another form of proof of citizenship, such as a birth certificate. Nowadays it's all about the passport. U.S. citizens reentering the United States by land or sea are required to have documents that comply with WHTI (Western Hemisphere Travel Initiative), most commonly a U.S. passport, a passport card, a trusted traveler card (such as NEXUS, SENTRI, or FAST), or an enhanced driver's license. The U.S. passport card is smaller than a traditional passport (think wallet size), cheaper, and valid for just as long, but you can't use it for travel by air.
Upon entering Mexico, all visitors must get a tourist card. If you're arriving by plane from the United States or Canada, the standard tourist card will be given to you on the plane. They're also available through travel agents and Mexican consulates and at the border if you're entering by land.
You're given a portion of the tourist card form upon entering Mexico. Keep track of this documentation throughout your trip; you will need it when you depart. You'll be asked to hand it, your ticket, and your passport to airline representatives at the gate when boarding for departure.
If you lose your tourist card, plan to spend some time (and about $30) sorting it out with Mexican officials at the airport on departure.
A tourist card costs about $20. The fee is generally tacked on to the price of your airline ticket; if you enter by land or boat you'll have to pay the fee separately. You're exempt from the fee if you enter by sea and stay less than 72 hours, or by land and do not stray past the 26- to 30-km (16- to 18-mi) checkpoint into the country's interior.
Tourist cards and visas are valid from 15 to 180 days, at the discretion of the immigration officer at your point of entry (90 days for Australians). Americans, Canadians, New Zealanders, and the British may request up to 180 days for a tourist card or visa extension. The extension fee is about $20, and the process can be time-consuming. There's no guarantee that you'll get the extension you're requesting. If you're planning an extended stay, plead with the immigration official for the maximum allowed days at the time of entry. It will save you time and money later.
Mexico has some of the strictest policies about children entering the country. Minors traveling with one parent need notarized permission from the absent parent. And all children, including infants, must have proof of citizenship (the same as adults) for travel to Mexico.
If you're a single parent traveling with children up to age 18, you must have a notarized letter from the other parent stating that the child has his or her permission to leave the country. The child must be carrying the original letter—not a facsimile or scanned copy—as well as proof of the parent/child relationship (usually a birth certificate or court document), and an original custody decree, if applicable. If the other parent is deceased or the child has only one legal parent, a notarized statement saying so must be obtained as proof. In addition, you must fill out a tourist card for each child over the age of 10 traveling with you.
Mexican Embassy (202/728–1600. embamex.sre.gob.mx/eua/.)
U.S. Department of State (877/487–2778. travel.state.gov.)