Identification

Adults (age 16 and over):
A current passport book or card are required.

A US Passport  A US Passcard

Children & Minors:

See our page on Bringing your Children

More information is available at the following links:
Note that visiting Estado29 is a "Land Travel" type of trip.

US Dept. of State  - The best official list of accepted documentation
Homeland Security - More details. 
If you hold a green card, check: US Customs & Border Protection