After any trip outside the United States (no matter how short), U.S. lawful permanent residents need to be ready to show U.S. Customs and Border Protection (CBP) officers—in addition to their actual permanent resident card ("green card")—evidence that they have not abandoned their status in the United States. This is to overcome a basic legal problem: After abandoning one's U.S. residence or being away for one year or more, the green card is no longer considered a valid travel document.
(Also see How to Travel Outside the U.S. With a Green Card for practical tips.)
The need to prepare evidence of U.S. ties is especially important for green card holders who anticipate lengthy or frequent absences. In fact, green cards automatically become invalid for entry after absences of one year or longer. In such circumstances, most green card holders (excluding refugees, asylees, and crewmembers) need to meet a special documentary requirement: They must apply for a reentry permit in advance, without which they are likely to be placed in removal proceedings upon return.
WARNING: A reentry permit is not an option for people who are already outside the U.S., for example who become stranded abroad owing illness or unexpected travel restrictions. The permit can be applied for only from within the U.S., in advance of a trip. There are other means of reentry you can seek, however, as described next.
Regardless of whether you fill out any formal requests, be sure to gather evidence that you've kept your immigration status in the U.S. ahead of time, before you attempt to return. Such evidence can include, for example, leases, rental agreements, or mortgage documents showing that you maintained a home or other property in the United States; proof of the presence of family members in the U.S., such as school reports and proof of employment; as well as documentation of any other ties to the United States.
Most green-card holders (excluding refugees, asylees, and crewmembers) need a reentry permit in order to enter the U.S. after any expected absence of one year or more.
There are two notable exceptions: current or recent employees of the American university in Beirut, Lebanon, and current or former employees of the U.S. government who traveled abroad in the course of their duties (or the spouse and children of such employee, if entering the U.S. within four months of the employee's return). For these two classes of green card holders, traveling on an "expired" green card normally satisfies the documentary requirement for reentry.
Beyond that, if you are a green card holder who finds yourself abroad for a year or more due to circumstances beyond your control and through no fault of your own, your first and best option would be to apply for a returning resident (SB-1) visa, as described on the Returning Resident Visas page of the State Department's website.
Your second, although riskier option is to find your way to a U.S. border or other point of entry and ask the customs officers to let you in (as a returning resident) in the exercise of their discretion—for "good cause" (meaning because you have a good excuse for not presenting a reentry permit or visa). You would request such an exercise of discretion by filing Form I-193, Application for Waiver of Passport and/or Visa, issued by U.S. Citizenship and Immigration Services, or USCIS.
Neither of these two options will likely work, however, if you had time to anticipate the length of your absence.
If eligible, you can request a reentry permit by filing Form I-131, Application for Travel Document (available from USCIS). Check box 1.a in Part 2 of the form, complete Parts 1 through 5 and 8, and send it to USCIS with a copy of the front and back of your green card and the required fee. If you can, attach some evidence to document the reasons for your intended absence.
After filing Form I-131, you should be scheduled for a biometrics appointment (to get your fingerprints and photograph taken) if your age is between 14 and 79. (If that is the case, be sure to submit your own photos if you wish to avoid the biometrics appointment; the photo is still required of you, just not the fingerprint check.)
Make sure you are present in the United States to attend the appointment—subsequent to which you could (if you requested it) pick up the reentry permit at a U.S. embassy or consulate abroad. (You could also request expedited processing if you have an emergency.)
If your reentry permit application is denied, you can appeal the decision by filing USCIS Form I-290B, Notice of Appeal or Motion.
Keep in mind that even obtaining and complying with a reentry permit does not guarantee that you will be allowed to keep your U.S. permanent residence. Your readmission under that status would still depend on whether you are able to present evidence of ties to the United States.
Extended absences (of two years or more) might require consecutive applications for reentry permits. (Reentry permits are valid for no more than two years and cannot be extended.)
Consecutive applications are allowed, but USCIS might look upon them with particular suspicion. In fact, the validity of reentry permits is limited to one year for most green card holders who have been outside of the U.S. for more than four years since obtaining their green card.
Nonetheless, this limit does not apply to active professional athletes and employees of international organizations, the U.S. government, or the American University in Beirut. (Although, as noted earlier, the latter might not normally need a reentry permit, they may choose to obtain one in certain circumstances—for example, when they might not return to the U.S. immediately following the conclusion of their work abroad.)
If you are planning an extended absence from the United States, or have become stranded outside the U.S., you would greatly benefit from the assistance of an experienced immigration attorney in devising a strategy to facilitate your reentry into the country.