As of January 31, 2007 everyone entering the United States is required to have a current, valid passport. If arriving by air, sea or highway. This is a huge change in document requirements as many Americans have in the past only needed a birth certificate and a driver's license.

Below is the website for the United States Department of state and has all of the up todate details, forms, and requirements for getting a US passport:

travel.state.gov/passport/passport_1738.html