Originally Posted by Efrem
The majority of U.S. citizens - I forget the percentage, but it's surprisingly high - don't even have a passport.
As of today U.S. citizens don't need a passport to re-enter from Canada, Mexico or the Caribbean, though it can be helpful in establishing that one is a citizen. That's considered a security hole and is supposed to change soon. (Whether the country one is going to will let a U.S. citizen in without one is a separate question, over which the U.S. has no control.)
Don't have back-up, but read somewhere recently that only about 10% of US citizens have passports (I think that is a little low, but probably not far off the mark!). That said, a passport is the best form of ID you can have and highly recommend everyone get one as pretty soon you will not be able to re-enter the US without one (you may get out, but they won't let you back in!). See
www.travel.state.gov. Currently a US citizen only needs "proof of citizenship" to re-enter the US from Canada, Mexico and much of the Caribbean. "Proof of citizenship" is NOT a drivers license or your social security card. Proof is a birth certificate (state issued, with the raised seal), citizenship papers, naturalization papers....
Welcome to the world of travel, may it prove as satisfying an experience to you as it does for the rest of us!!