If you are a permanent resident (Green Card holder) and are planning to travel outside the U.S. and remain outside for more than 6 months, you must get a re-entry permit to keep your Green Card valid which would prevent issues to arise on re-entering into the U.S.

1. Why you need a re-entry permit?
A re-entry permit can help preventing two types of problems:
(1). Your Permanent Resident Card became technically invalid for re-entry into the United U.S. if you are absent from the U.S. for 1 year or more;
(2). Your U.S. permanent residence being considered as abandoned for taking up residence in another country more than 1 year. A re-entry permit establishes a presumption that you did not abandon your status, and it allows you to apply for admission to the U.S. after traveling abroad for up to 2 years, without having to obtain a returning resident visa.

2. Procedures
(1). File I-131 form to USCIS along with supporting documents and payment
(2). 2 to 3 weeks, you’ll receive I-797 notice
(3). About one month later, you’ll get your notice to get ten-printed
(4). After all above, your re-entry permit will be delivered to your whereabouts
Usually, it takes approximately 3 months to process, please see the latest processing time at USCIS official website at http://www.uscis.gov/portal/site/uscis 

3. Filing Material
Form I-131,Application for Travel Document
Supporting Documents: A copy of your Permanent Resident Card , a copy of the biographic page(s) of your passport if you have not yet received your Form I-551, a copy of the visa page showing your initial admission as permanent resident, a copy of the Form I-797, Notice of Action, approval notice of an application and so on.

4. Tips
(1). A Re-entry Permit has two-year period of validity and it may not be extended;
(2). If you do not obtain a Re-entry Permit and remain outside the U.S. for one year or more, it may be determined that you have abandoned your permanent or conditional resident status;
(3). Departure from the United States before a decision is made on an application for a Re-entry Permit usually does not affect the application.
(4). Departure from the United States before the biometrics are collected, the application will be denied;
(5). If you are under 14 years of age, your parent or guardian may sign the application on your behalf.