To enter Mexico, all U.S. citizens must have undamaged passports or passport cards that are valid for at least six months. Additionally, all travelers going beyond the Mexican border zone must obtain entry permits called Forma Migratoria Multiple, or FMM, at or just inside the border. Travelers must surrender their FMMs when they leave Mexico, and U.S. citizens must show their passports when they re-enter the United States.
Although Mexican authorities accept U.S. passport cards for entry by land, sea and air, to return to the United States by air, U.S. citizens must have full valid passports. Passport cards are acceptable for return by land or sea only. Tourists traveling by land in the 20 to 30 kilometer border zone within Mexico for three days or less do not need FMMs. All other travelers can obtain FMMs for a fee at border crossings, Mexican consulates, Mexican tourist offices and airlines that fly into Mexico. When obtaining their FMMs at borders or within Mexico, tourists should be aware of prevalent scams involving bogus documents and deal only with valid Mexican immigration officials within their offices.
Travelers should safeguard their documents and carry photocopies of their passports and FMMs. If they lose their documents, Mexican officials may fine or arrest them. Regulations on travel documents also apply to U.S. citizens who enter Mexican territorial waters for fishing or other purposes. Minors 18 years and under traveling without their parents need additional documentation.