Before coming to the United States, most people from other countries must first obtain a visa. A nonimmigrant visa is used for temporary stays to visit, study, or work. An immigrant visa is the one used to enter the United States after approval for permanent residence (a "green card").
All U.S. visas, however, are issued on the condition that the holder abide by their terms. For example, the visa holder must not engage in activities outside of those permitted (tourists may not work, for instance), and the person must leave the United States within the time required.
If you don't comply with the terms of the visa, it may be cancelled at any time, whether before, during, or after your stay in the United States.
Sometimes a visa is cancelled before a person's trip because the U.S. government obtains evidence that the person plans to use the visa for a different purpose than was intended; for example, to stay in the U.S. permanently instead of making a short visit. Or a visa may be revoked when a person goes to a U.S. consulate to apply for a new visa, and the officer discovers that the person misused the old visa.
Sometimes, however, visa cancellation is simply an administrative matter -- for example, the consular officer needs to cancel an old visa before authorizing a new one.
A common reason for visa revocation is that the holder stayed in the United States longer than was allowed. Visitors to the United States are often confused, and think they're allowed to stay in the U.S. until the expiration date on the visa. But that date is only the last date upon which the person can use the visa as a U.S. entry document.
The date by which you must actually leave the United States is shown on your Form I-94 Arrival/Departure Record. If you stay even one day past that date, without having applied for an extension or change of status, your visa is said to automatically cancel.
If your visa is cancelled, you will either need to leave the United States immediately or, if you're in another country, delay your travel plans until you have successfully applied for a new U.S. visa. Depending on the reasons for the visa cancellation, however, you may be refused additional entry visas.
If your visa is revoked, or you believe you may be at risk of an overstay or visa cancellation, contact an experienced U.S. immigration attorney. Your lawyer can help you evaluate your situation, perhaps take steps to find out why your visa was canceled, and help make sure that the next time you apply to come to the US, you have the best possible chance of success. See our section on hiring an immigration lawyer for more information about the cost and benefits you can expect from an attorney.