A visitor who comes to the U.S. lawfully from another country typically uses what's called a "nonimmigrant visa." For instance, it might be a K-1 visa for the fiance of a U.S. citizen, an M-1 visa for a vocational program student, or a B-2 visa for a tourist. In every such case, the person will, upon entering the U.S., be told the date by which he or she must depart, and that information will be recorded online, in a form known as an I-94.
But what happens if someone stays in the U.S. beyond that permitted date? U.S. immigration law builds in various consequences for people who overstay their visa or permitted U.S. stay.
Your period of authorized stay in the U.S. is the amount of time you are allowed to remain in the country. As mentioned, it is found on your I-94 document. At one time, I-94s were issued on paper cards, but they are now, in most cases, accessible via the I-94 page of the U.S. Customs and Border Protection (CBP) website.
Depending on the type of visa you have and its terms, you may be able to extend your stay. For example, if you are in the U.S. for a job, and your employer wants to continue hiring you, and extensions are allowed on your visa, you could apply for one. Extension applications must be submitted to U.S. Citizenship and Immigration Services (USCIS) before the expiration of your period of authorized stay. (In most cases, extensions are requested by filing USCIS Form I-539.)
What if you file for an extension but USCIS waits to approve it until after the end date of your period of authorized stay? (This is not an unlikely scenario; USCIS is often backed up with applications.)
If you filed your extension application on time (before your authorized stay period ran out), and USCIS eventually approves it, your period of stay will automatically be extended from the previous end date. This happens regardless of whether USCIS approves the extension before or after the end date. This way, you don’t end up with any gaps in your period of authorized stay.
If USCIS does not approve your extension application, however, you will be expected to leave the U.S. immediately.
It's unlikely, but possible that you could be apprehended by U.S. immigration authorities when overstaying. (It would, however, take a huge squad of officials to chase after every U.S. entrant to make sure they leave on time.)
If you were apprehended, however, you would be placed into removal proceedings. In the absence of any defense to deportation, you would be ordered removed from the U.S., and barred from returning for a number of years.
If you overstay your period of authorized stay—even by one day— then you trigger what is known as the consular shopping bar. This comes from Section 222(g) of the Immigration and Nationality Act (I.N.A.), which states that if you remain inside the U.S. beyond your period of authorized stay, your visa will be cancelled and no longer valid for entry. Additionally, if you want to apply for another visa to the U.S., you will be required to do so at the U.S. consulate in your country of nationality.
Before the consular shopping bar was enacted, many foreign nationals who overstayed simply went to consulates in Mexico or Canada to apply for another period of stay or a new visa. But now, unless you can show that extraordinary circumstances apply to your case, you must reapply for a visa at the consulate located in your country of nationality.
Also, if you applied for an extension and were denied, you won’t trigger the consular shopping bar so long as you had good reasons for requesting an extension; that is, your extension request was not frivolous.
Let's say you became eligible for a U.S. green card, for example by marrying a U.S. citizen or receiving a job offer from a U.S. employer. You're thinking it would be convenient to take care of all your paperwork in the U.S., without leaving, using a procedure known as "adjustment of status."
Realize, however, that most applicants must have entered the U.S. with permission, after inspection by border agents, and be in valid visa status at the time of their application if they are to adjust status. This includes a requirement that you have not have stayed past the expiration of your permitted stay or worked without permission from U.S. immigration authorities. That doesn't mean such applicants can never get a green card. Leaving the U.S. and applying through what's known as "consular processing" is still a potential option, though could be complicated by the unlawful presence inadmissibility bar described below.
Also, there's an important exception for the immediate relative (children, spouse, or parents) of U.S. citizens. As long as they entered in valid visa status (and didn't use that visa fraudulently with the intent to apply for a U.S. green card after arriving), they may use "adjustment of status" as the procedure by which to apply for their green card even after an overstay.
As of a 1997 change in the law, anyone who stays continuously in the U.S. without a proper visa for more than 180 days but less than 365 days and then leaves is barred from returning to the U.S. for three years. Worse yet, someone who stays illegally for a year or more and then leaves cannot come back for ten years.
This doesn't affect everyone. First, someone who was under age 18 for any of the time in the U.S. unlawfully need not count that time toward these bars. Nor does any illegal time before April 1, 1997, or for less than 180 continuous days (even after April 1997) count. Other, narrower exceptions or special situations exist. And it's possible to ask for a waiver of this bar.
For more information, see Consequences of Unlawful Presence in the U.S.—Three– and Ten-Year Time Bars. And you'll want to get help from an immigration attorney in analyzing such complicated matters and preparing a waiver application.