www.travel.state.gov

AIR TRAVEL

ALL PERSONS traveling by air outside of the United States are required to present a passport book or other valid travel document to enter or re-enter the United States.

LAND AND SEA TRAVEL

ALL PERSONS traveling by land and sea outside of the United States are required to present a passport book/card, or other valid travel documents to enter or re-enter the United States.

The following summarizes information available on the Department of Homeland Security’s website:

• Effective June 1, 2009, all U.S. citizens are now required to present a passport book, passport card, or WHTI-compliant document when entering the United States.
  Please Note: Children under age 16 will be able to continue crossing land and sea borders using only a U.S. birth certificate (or other form of U.S. citizenship such as a naturalization certificate.). The original birth certificate or a copy may be used. See the Department of Homeland Security's Ready, Set.. Go! for more information on the changing travel requirements. 

The passport requirement does NOT apply to U.S. citizens traveling to or returning directly from a U.S. territory.