If you have U.S. lawful permanent residence, you will have been issued a photo identity card proving your status, known as a "green card." That card has an expiration date. The time allotted before your card expires is 10 years. After this time has passed, the green card holder is expected to renew the card (using Form I-90, issued by U.S. Citizenship and Immigration Services or USCIS).
But what if you are in the process of applying for U.S. citizenship when the green card expires?
A green card is merely evidence of your lawful immigration status. You are still a permanent resident, even if your green card has expired. On that basis, many people have chosen to simply apply for U.S. citizenship after their green card has expired, or to not renew the green card if it happens to expire while they're waiting for a USCIS decision on their citizenship application.
USCIS policy goes back and forth on how accepting it is of this practice, but lack of a renewed green card should not, by itself, hold up your naturalization approval.
Normally, you are expected to carry a valid green card with you at all times, as one of the responsibilities of being a permanent resident. Meanwhile, applying for citizenship or naturalization can be a months-long process. (The exact timeline depends on the USCIS office at which you apply.)
Spending all that time without a green card could make it difficult to gain employment in the U.S. (or prove your ongoing legal status to your current employer). It will also make it difficult to reenter the United States if you travel abroad. Then again, the USCIS processing time to renew a green card also tends to be long; over a year is not uncommon.
The process of renewing a green card involves submitting Form I-90 to USCIS, along with a copy of your green card and a fee. You'll also need to attend a biometrics appointment. For details, see How to Renew Your U.S. Green Card.
If you really don't want to renew your green card, then you're on safest ground if the date that you apply for naturalized citizenship is more than six months prior to the date your green card will expire. And you'll want to make sure you have a valid form of photo identification when you attend your naturalization interview, such as a passport from your home country, since you won't have a green card to use as ID.
If you've submitted your naturalization application then decide you don't want to risk waiting without a green card for a decision, you can also submit an I-90 renewal application and have the two applications run on separate tracks. If your green card renewal is still pending when you're called in for the naturalization interview, bring the receipt notice from USCIS.
USCIS does offer a backup for of proof of permanent residency, known as an I-551 or ADIT stamp. To obtain one, you can arrange an appointment at a USCIS office, either by calling USCIS's Contact Center or requesting the appointment online. This stamp will be placed into your passport or I-94, and will serve as temporary proof of your LPR status while you await a renewed green card or approval for naturalized citizenship.
If you're concerned about any legal incidents that could cause your application for green card renewal or naturalized citizenship to put your legal status at risk, talk to an immigration lawyer before you proceed. Have an attorney review your recent history, legal permanent resident status, and full immigration files to make sure you’re not getting yourself into hot water with USCIS or the Department of Homeland Security.