ENTRY REQUIREMENTS: The Government of Mexico requires that all U.S. citizens present proof of citizenship and photo identification for entry into Mexico. A U.S. passport is recommended, but other U.S. citizenship documents such as a certified copy of a U.S. birth certificate, a Naturalization Certificate, a Consular Report of Birth Abroad, or a Certificate of Citizenship are acceptable. U.S. citizens boarding flights to Mexico should be prepared to present one of these documents as proof of U.S. citizenship, along with photo identification. Driver's permits, voter registration cards, affidavits and similar documents are not sufficient to prove citizenship for readmission into the United States.
U.S. citizens do not require a visa or a tourist card for tourist stays of 72 hours or less within "the border zone," defined as an area between 20 to 30 kilometers of the border with the U.S., depending on the Mexican state. U.S. citizens traveling as tourists beyond the border zone or entering Mexico by air must obtain a tourist card, also known as an FM-T, available from Mexican consulates, Mexican border crossing points, Mexican tourism offices, airports within the border zone and most airlines serving Mexico. The tourist card costs approximately $20.00; the fee for the tourist card is generally included in the price of a plane ticket for travelers arriving by air.
(Taken from the Department of State website)