If you're renting a car on your next world trip, you may need an additional ID: an international driving permit. Not paying for one, which translates your license into 10 languages, could be an expensive mistake.

That's the conventional wisdom.

But people like Bill Oliveri have begun to question that. He contacted me recently before his trip to Italy to find out if the $20 he'd paid AAA for a one-year international driving permit was money thrown away.

"There's always the fear that the car-rental company will not honor my reservation," says Oliveri, a retiree from Chevy Chase, Md. "So I pony up the $20 and wait in line at AAA."

International driving permits may be one of the least-understood travel documents. It's a gray booklet with your name and photo and a translation of your license into French, Italian, Swedish, German, Chinese, Arabic, Spanish, Japanese, Russian and Portuguese. The State Department authorizes AAA and the American Automobile Touring Alliance to issue international driving permits.

Julie Hall, a spokeswoman for AAA, says most countries "highly recommend" an international driving permit, and some require it.

"There are some countries that do not recognize a driver's license from the United States, but officially recognize an international driving permit, like Hungary," she says. "There are other countries that honor a United States driver's license, but require a local language translation, like Spain."

You can obtain an international driving permit at any AAA branch office. You need two passport photos and a valid American license, and there's a short form to fill out. AAA will also process your application by mail. You don't have to be a member.

But is a permit necessary? Ask people who have rented cars overseas, and you'll receive a variety of responses.

Charles Breitbart, who publishes the travel guide TripTins.com, says you're better safe than sorry.

"For any long-term traveler or for someone who plans to travel and rent a car several times a year, I believe it is a no-brainer to just head to AAA and pay $20," he says. "It removes all worry when renting a car."

He says you never know when you'll need a permit.

"When I visited New Zealand they were fine with just my regular license. But when I visited South Korea, they required the international one as well. It simply just depends on a country-by-country basis."

So how do you find out who needs an international driving permit? Start by asking your car rental agency or travel agent. In Europe, Austria, Bosnia, Croatia, Germany, Greece, Hungary, Italy, Poland, Romania, Slovenia, Slovakia and Spain require an international driving permit, according to the car rental agency Auto Europe.

Bottom line: If you're headed to the most popular tourist destinations in Europe, you may not need a permit. But you might want to get one, just to be safe.

Christopher Elliott is the ombudsman for National Geographic Traveler magazine. Read more tips on his blog, elliott.org, or e-mail him at chris@elliott.org.