Required Documents for Crossing Back into the US.

GENERAL REQUIREMENTS:

INDIVIDUAL IDENTIFICATION

You will need to prove two things, who you are, and where your citizenship is. This can be done in two different ways, the one document option or the two document option.

The one document option;
One of the following documents should be presented to prove both identity and citizenship.
AS OF JUNE 1, 2009 THIS IS THE ONLY OPTION!

Acceptable Documents as of January 31, 2008
• U.S. or Canadian Passport
• U.S. Passport Card (Available now)*
• Trusted Traveler Cards (NEXUS, SENTRI, or FAST)*
• State or Provincial Issued Enhanced Driver's License (when available - this secure driver's license will denote identity and citizenship.)*
• Enhanced Tribal Cards (when available)*
• U.S. Military Identification with Military Travel Orders
• U.S. Merchant Mariner Document
• Native American Tribal Photo Identification Card
• Form I-872 American Indian Card
• Indian and Northern Affairs Canada (INAC) Card
* Frequent Land Border Crossers- To expedite processing into the United States, U.S. Customs and Border Protection recommends using one of the above asterisked documents.

The two document option.
NOTE: The Two Document Option is no longer accepted as of June 1, 2009. From that date on, only the One Document Option is allowed. Therefore, I've deleted the rest of the two document option.

The one exception is the States that have complied with the WHTI guidelines, the drivers license from these States is also a proof of citizenship. These licenses are called “REAL ID”. Currently the only states that I know of that issues WHTI compliant driver's license are Michigan, New York, Vermont and Washington. Arizona signed the “REAL ID” Act of 2005 agreement in 2007 but I don’t believe they are issuing the REAL ID yet.
NOTE; REAL ID is NOT valid for international air travel!

CHILDREN UNDER 18
U.S. and Canadian citizen children ages 18 and under will be expected to present a birth certificate issued by a federal, state, provincial, county, or municipal authority.
If both parents aren't traveling together, a notarized letter from the missing parent granting permission for you to take the kids into Mexico is required. A form for this is available at Parental Affidavit. Make sure you get it notarized!

A complete discussion of issues, such as travel documents, insurance, pets, and medical care, is available here.