After you have immigrated to the United States and received a green card (become a lawful permanent resident or LPR), travel gets a lot easier. No longer do you have to worry about whether your visa will be good for another U.S. entry, or whether it has expired.
On the other hand, there are specific rules you must follow in order to maintain your U.S. immigration status.
That may sound obvious, but your green card is a crucial document for you to carry in order to reenter the United States. In fact, you should make a copy of it and leave one copy in your suitcase while traveling, and another with a friend in the U.S. whom you would call in an emergency. These will help you prove your identity as a green-card holder in the event that your card gets lost or stolen. (You would, in that case, want to go to the nearest U.S. embassy or consulate.)
You will also need to carry the passport of your home country, since this is a required travel document and shows your country of citizenship.
The purpose of acquiring a green card is to establish U.S. residence and possibly work toward qualifying for U.S. citizenship. But if you leave the U.S. with the intention of making your home elsewhere, you will potentially give up your U.S. residency. One way that this will be discovered is if you spend long periods of time outside the United States -- though, contrary to popular rumor, simply coming back to the U.S. every six months or a year is not enough. You can lose your U.S. residency in a day if you leave with the intention of moving to another country. Factors that will be considered include where you maintain your home, where you work, where your car is registered, where your family lives, and so on.
What is true, however, is that if you stay away for six months or more, you will face a lot of questions upon your return -- and if you stay away for a year or more, you will be presumed to have abandoned your U.S. residency.
Another important thing to know is that continuous residence in the United States is a requirement for citizenship, as discussed below.
(You can find more information in the article we have listed under Abandonment of Residence.)
When you return to the U.S., the Customs and Border Protection (CBP) officer who greets you will ask questions about how long you were away, what you were doing, where you live in the United States, and so forth. If your answers don't sound right -- for example, you've been working abroad and are back for a few weeks to stay with a friend -- the officer may refer you for secondary inspection. There, another officer will question you further about your ties to the U.S., search your bags, and so on.
If you clearly have given up your U.S. residency, you may be turned around and sent home on the next flight. More likely, however, you will be "paroled" into the U.S., meaning allowed temporary, conditional entry for the purpose of proving that you didn't abandon your residency. If you fail to provide this proof, you will be deported (removed) from the United States.
Most people can apply for naturalized U.S. citizenship five years after becoming a lawful permanent resident. (That time period reduces to three years if you have been married to and living with a U.S. citizen the whole time.) However, half of those five or three years must have been spent inside the United States before you can become eligible. When you apply for citizenship, you will be asked about all your trips outside the U.S., and those time periods will be taken into account in determining whether you are yet eligible to apply.
Another important concept with regard to eligibility for citizenship is "continuity of residence." If you spend more than six months but less than one year outside the U.S., you may be found to have disrupted your continuous residence, unless you can prove otherwise. What's more, if you spend at least one year outside the U.S., you will be automatically found to have disrupted your continuous residence, and will have to wait to file your application for naturalization until four years and one day following the date of your return to the United States to resume your permanent residence.
All of this sounds a lot like the principles of abandonment of residence described above, but it's a separate section of the law. You can break the continuous residence requirement without having actually abandoned your U.S. residence. (Of course, if you have abandoned your U.S. residence -- which sometimes gets discovered in the course of applying for U.S. citizenship -- you will have no hope of citizenship and will lose your green card.)
If you know in advance that you are going to remain outside the U.S. for more than one year, but less than two years, you can apply for what's known as a reentry permit, before you leave. If you are out of the United States for longer than a year, you will not be able to use your green card as a reentry permit, and will more than likely lose it completely.
The reentry permit creates a presumption that you are not abandoning your immigration status nor your permanent residence by being out of the country for a protracted period of time. It also allows you to apply for admission to the U.S. after traveling abroad for as long as two years, without having to obtain a returning resident visa.
As mentioned above, it is best to file this application well in advance of leaving on your trip. You do not need to have received the reentry permit before leaving, however. For instructions, and further information, see Reentry Permit Process for U.S. Permanent Residents.