If you are a citizen of another country and have been living in the U.S., you may need special documents if you leave the U.S. and then return.
If you are a permanent or conditional permanent resident who has been away from the U.S. for less than one year, you will only need to show your Green Card upon re-entry to the U.S.
If you are a permanent or conditional permanent resident who has been outside the U.S. for one year or longer, apply for a re-entry permit before you travel. Use Form I-131 - Application for Travel Document.
If your re-entry permit has been lost or stolen, you may be able to re-enter the U.S. by applying for carrier documentation. Use Form I-131A - Application for Travel Document (Carrier Documentation).
If you are a foreign citizen re-entering the U.S., the documentation you need may depend on your immigration status:
You must apply for advance parole or a refugee travel document before you leave the U.S. If you do not, you may not be allowed to return to the U.S. Use Form I-131 - Application for Travel Document.