A valid U.S. passport can be used as proof of identification and is required for international travel. Most countries that require U.S. citizens to acquire a visa for entry require that the passport have at least six months of validity remaining to grant the visa. So, even if your passport's not yet expired but has less than six months remaining, you'll have to apply for a new booklet before you travel.

Recently Expired

You can renew your passport by mail if it's about to expire or expired within the past 15 years; you can also renew by mail if your name hasn't changed or the booklet hasn't been damaged within that time frame. Download a passport renewal application from the U.S. Department of State website. Provide the biographical information requested on the form, including your name, address and date of birth. You'll be required to submit a new passport photo along with the renewal application. Send the application, your new photo and your current or recently expired passport to the State Department or to your passport agency of choice, along with the fee. The State Department will issue you a brand new passport booklet and send the old one back to you -- invalidated -- to keep or discard.

Expired More than 15 Years

If your passport has been expired for 15 years or more, you'll have to renew it in person; you'll also have to renew it in person if it's been damaged. When such is the case, fill out the form, have a passport photo taken and drop it off at a passport processing center, usually your main post office or local passport agency. You'll also be required to present proof of citizenship and a government issued identification, such as a valid driver's license. The State Department will mail you a new booklet featuring your updated photo and information; you'll get the old one back to keep or throw out.

Expedited Passports

You may need to travel outside the U.S. on short notice. If your passport is expired, you'll get a new booklet, even if you request expedited service. You'll have to send in a new passport photo, which you can have taken and printed in a few minutes at most drug stores or business copy centers.

New Passport Technology

If you acquired your U.S. passport before 2007, your booklet isn't biometric, and you'll receive such a booklet when your current passport expires. Biometric passport booklets contain electronic features, including a chip, that help airport security keep track of passengers using facial recognition, retinal scans and fingerprints, according to the State Department. Biometric passports are designed to minimize the instances of passport fraud known to compromise air travel safety. You're permitted to use your nonbiometric passport until it expires.