In the past, it was possible for Americans to drive into Mexico or Canada using only a standard state-issued driver license and a birth certificate. That has changed—only kids can do that now. The Department of Homeland Security (DHS) and the Department of State jointly created the Western Hemisphere Travel Initiative (WHTI), a new system that they claim allows the government to keep a closer eye on who is crossing American borders.

U.S.-Mexico checkpoint at San Ysdiro, California

Now drivers need one of these to cross the U.S.-Mexico border by land:

U.S. citizens can present a valid U.S. passport or passport card (click here to find out how to apply for those); an Enhanced Driver’s License (which is a new type of license that proves your citizenship—click here for information about those); a card from one of the Trusted Traveler Programs (NEXUS, SENTRI or FAST)
• You can also use one of these niche forms of identification, if you qualify: a U.S. Military identification card as long as you are traveling on official orders, a U.S. Merchant Mariner document if you are traveling in conjunction with official maritime business, a Form I-872 American Indian Card, or (if it's available) an Enhanced Tribal Card.
• U.S. and Canadian citizen children under the age of 16 (or under 19, if traveling with a school, religious group, or other youth group) need only present a birth certificate or other proof of citizenship. The birth certificate can be original, photocopy, or certified copy.
• U.S. Lawful Permanent Residents are required to present their permanent resident card (Form I-551) or other valid evidence of permanent residence status.
Canadian citizens can present a valid passport, Enhanced Driver’s License, or Trusted Traveler Program card (NEXUS, SENTRI or FAST).
Bermudian citizens are required to present a valid passport.
Mexican citizens, including children, are required to present a passport with visa, or a Border Crossing Card.

If you are going to Mexico or Canada on a cruise that begins and ends in the United States and you're a U.S. citizen, you might be able to get away with using only a government-issued I.D. and your birth certificate—but don't count on it. Ask your cruise line before you leave home to make sure you won't need a passport.

If you have an Enhanced Driver's License, bear in mind that not all border crossings have the capability to read them. 

Our advice: Get a passport. It's the one document, it seems, that no border guard will refuse if it's valid, and it's useful to keep one around as identification during your non-vacation life. You'll also need a passport if you're flying into either country—no driver licenses are accepted as proper identification when you fly internationally.

Photo credit: Josh Denmark/ U.S. Customs and Border Protection