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U.S. and Canadian citizens entering Mexico are required to show a passport when visiting the country by international flight.

U.S. Citizens
Under the Western Hemisphere Travel Initiative, special rules apply to U.S. citizens traveling to Mexico, Bermuda and certain destinations in the Caribbean*.

U.S. citizens traveling by air are required to have a passport to re-enter the States. Anyone traveling by land or sea needs to present either (a) a passport, passport card or WHTI-compliant document; or (b) a government-issued photo ID, such as a driver’s license, along with proof of citizenship, such as a birth certificate.

As of June 1, 2009, most U.S. citizens entering the United States at sea or land ports of entry are required to show a passport, passport card, or WHTI-compliant document. They will no longer be allowed to use a government-issued photo ID as proof of citizenship.

The U.S. government recommends traveling with a valid U.S. passport to avoid delays or misunderstandings while visiting other countries. A lost or stolen passport is also easier to replace when outside of the United States than other evidence of citizenship. 

For the latest entry requirements, contact the Embassy of Mexico web site at http://portal.sre.gob.mx/usa/ or contact the Embassy of Mexico at 1911 Pennsylvania Avenue NW, Washington, DC  20006, telephone (202) 736-1000 or any Mexican consulate in the United States for the most current information.

Canadian Citizens
Canadians are required to carry a passport to enter Mexico. However, in order to avoid delays and misunderstandings, it is recommended that you travel with a passport valid for six months after your arrival date. It is the only universally accepted identification document, and it proves that you have a right to return to Canada. A lost or stolen passport is also easier to replace when outside of Canada than other evidence of citizenship.

These requirements are subject to change at any time. It is the traveler's responsibility to check with the country's embassy or its consulates for up-to-date information.

Sources
U.S. State Department: http://travel.state.gov/travel/
Canadian Consular Affairs: http://www.voyage.gc.ca/consular_home-en.asp
Mexican Government: http://www.visitmexico.com/wb/Visitmexico/Visi_travel_tips_TA

* Anguilla, Antigua and Barbuda, Aruba, Bahamas, Bermuda, British Virgin Islands, Cayman Islands, Dominica, Dominican Republic, Grenada, Jamaica (except for business travel), Montserrat, Netherlands Antilles, St. Kitts and Nevis, St. Lucia, St. Vincent and the Grenadines, and Turks and Caicos.