Effective June 1, 2009, U.S. citizens entering the United States by land or sea (in addition to entry by air) must present a valid passport, passport card, or other travel document approved by the Department of Homeland Security. Other photo ID (such as a drivers license) and proof of citizenship (such as a birth certificate) will no longer be allowed. Children under the age of 16 who are U.S. citizens, however, will be able to present the original or copy of their birth certificate or other proof of U.S. citizenship such as a naturalization certificate or citizenship card. You can obtain more information at http://www.travel.state.gov/travel/cbpmc/cbpmc_2223.html.