If you are a foreign national and have been married to your U.S. citizen spouse for less than two years when you either arrive in the United States on an immigrant visa or are approved for a green card (adjustment of status) based on that marriage, you will begin life in America as a conditional, not permanent resident. (By definition, every immigrant who enters the U.S. on a K-1 fiancé visa and then applies to adjust status will start out as a conditional resident.)
Let's take a closer look at who this applies to and what it means.
In any marriage-based application for a green card, U.S. immigration authorities look very closely at whether the marriage is the real thing (bona fide), not just a fraud or sham to get the immigrant a green card. Due to concern that recently entered into marriages are more likely to be shams, the immigration laws basically gave U.S. Citizenship and Immigration Services (USCIS) a second chance to review the validity of the marriage, by initially making the new immigrant a "conditional," not "permanent" resident.
In most ways, conditional residents have the same rights as permanent residents. They can travel in and out of the U.S., accept employment without separately applying for a work permit, and start working toward U.S. citizenship. (Time as a conditional resident counts as permanent residence for purposes of applying for naturalization, so long as the immigrant is eventually approved for permanent residence.)
The key difference is that conditional residency expires after a two-year "testing" period. Not long before the expiration date, the immigrant will have to apply for permanent status. That application process involves proving that the marriage is ongoing, and supplying further proof (in addition to that already submitted over the course of the earlier application process) that the couple is truly establishing a life together.
In order to remove the conditions on residence, the immigrant and the U.S. citizen spouse must submit a joint petition on Form I-751 to USCIS. This form is available for free download on the USCIS website. The timing of submitting this form is very important: It must be sent within the 90-day period before the immigrant's conditional residence ends.
Also important is submitting documents to show the ongoing, real marriage, such as copies of joint leases, bank accounts, memberships, and more. Birth certificates of any children born to the marriage are particularly powerful evidence.
USCIS may, after reviewing the I-751, decide to interview you and your spouse, to take a closer look at whether your marriage is real. If USCIS determines that the marriage is not real, the immigrant could lose status entirely and have to leave the United States.
If the marriage has ended before the immigrant becomes a permanent resident, such as through death or divorce, or the U.S. spouse has become abusive, it is possible, under certain limited circumstances, for the immigrant to apply for a waiver of the joint petition requirement. The immigrant would submit the I-751 solo.
In addition to providing proof that the marriage was the real thing, the immigrant will have to show documentary evidence of the claimed basis for the waiver. Also see:
You'd likely want an attorney's help with any of these.
If the marriage is already two years old when the immigrant either arrives at the U.S. border with an immigrant visa or receives approval for a spousal green card, they will receive permanent residency and won't have to worry about the status expiring (although the actual card will need to be replaced once every ten years).
For questions on any of this, and help with the challenging task of preparing a waiver application, consult an experienced immigration attorney.