As of October 1, 2007, every citizen flying home to the U.S. needs a passport, even if you're returning from a quickie getaway to Mexico, Canada, Bermuda, or the Caribbean.
The rules are different if you're driving across the border (such as from Canada) or sailing a cruise ship. You won't need a passport for those trips—for a while. [UPDATE 1/3/2008: A new rule is expected to go into effect by summer 2009. That rule may require U.S. citizens present a passport or a passport card when crossing the Canadian border and when doing other land/sea border entries. Until then, you won't need a passport.]
Yet today at land and sea entry points, the best way to avoid border hassles is to show an original copy of a citizenship document, such as a birth certificate, and a government-issued photo ID, such as a driver's license. Just showing a passport will also do the trick. Citizens may soon be required to show these documents, if new rules go into effect, as expected, on January 31, 2008.
For most Americans, the biggest adjustment will be for travelers to Canada and for cruise-goers whose ships sail into international waters. In the past, you could re-cross the border by car from Canada, or disembark from a cruise ship at a U.S. port, simply by saying you were a citizen and answering some questions. Now the rules noted above apply. [No passport is required, however, when traveling to Puerto Rico because it's a U.S. territory.]
A note about children: U.S. citizens aged 15 and under who are traveling by air need passports. But if they're traveling by land or sea, passports are not necessary. A certified copy of the child's birth certificate is highly recommended, though. And if you're a divorced parent, you are advised to show documentation proving you have custody of the child.
Many post offices have been set up to accept passport applications. Bring your application to a a post office or official passport processing center, with proof of citizenship, photo ID, two photos, and $97. As a general rule, allow about three months from application to receive your passport. The Postal Service website offers a passport application.
One more note of caution: If you don't have enough blank pages in your passport, or if it expires in six months or less, some countries won't let you in. Check the rules at the website of the embassy of the country you're visiting. Find websites and more info by visiting state.gov/travel.
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