Most foreign nationals seeking entry to the  U.S. are required to possess both a valid passport and a valid U.S. visa to apply for admission to the U.S. “Automatic visa revalidation” allows a foreign national with an expired nonimmigrant visa (“NIV”) to apply for re-entry to the U.S using a valid, unexpired I-94 in place of a valid visa following a trip to Canada or Mexico.
The person seeking re-entry MUST fulfill the following factors.

(1) Be returning to the U.S. from a visit to Canada, Mexico, or an adjacent island* of less than 30 days duration. Note that this procedure is not available if any other countries are visited while the person is outside the U.S.

(2) Possess a valid, unexpired Form I-94 (and, in the case of F or J nonimmigrants, a current I-20 or DS-2019). This means that you cannot allow the airline (or anyone else) to remove your I-94 when you depart from the U.S.

(3) Possess a nonimmigrant visa. It may be expired, unexpired, and in the same or different visa category. However, the person seeking re-entry must not have violated or failed to maintain his or her status when previously in the U.S.

A nonimmigrant meeting these standards normally will be allowed to re-enter the United States as though he or she had never left the U.S. If the person seeking re-entry has had his or her status changed by USCIS, then the person will be readmitted in the changed classification as if the person had been issued an NIV in the proper category.
This is a very technical, though often beneficial, provision for travelers.  If you have concerns regarding entry to the U.S., contact our office to schedule a consultation.
*8 CFR 286.1(a) The term “adjacent islands” means Anguilla, Antigua, Aruba, Bahamas, Barbados, Barbuda, Bermuda, Bonaire, British Virgin Islands, Cayman Islands, Cuba, Curacao, Dominica, the Dominican Republic, Grenada, Guadeloupe, Haiti, Jamaica, Marie-Galante, Martinique, Miquelon, Montserrat, Saba, Saint Barthelemy, Saint Christopher, Saint Eustatius, Saint Kitts-Nevis, Saint Lucia, Saint Maarten, Saint Martin, Saint Pierre, Saint Vincent and Grenadines, Trinidad and Tobago, Turks and Caicos Islands, and other British, French and Netherlands territory or possessions bordering on the Caribbean Sea.