If you are a green card holder and will be outside the US for less than twelve months (one calendar year) you will need your green card. Otherwise, you will need to apply for and receive form I-131, which you will need to present to Customs and Border Protection (CBP) authorities before re-entering the country. Form I-131 will need to be presented at a port of entry.
There are many cases in which you may need Form I-131 to re-enter the US. For example, if you are awaiting a response on an application to adjust status to permanent resident (form I-485), you may need form I- 131 in order to avoid forfeiting your application. If you leave the country without Form I-131 while awaiting the completing of your Form I-485 application, you may be denied re-entry into the US and you may forfeit your application to adjust status to permanent resident. However, if you are applying to adjust status while maintaining H-1, H-4 or L-2 status, you may not need Form I-131 to re-enter the US.
If you need to enter the US for specific types of emergency humanitarian work, the United States Citizenship and Immigration Services (USCIS) may also require that you secure Form I-131 first. While Form I-131 will not help you avoid traditional visa applications, it can make entering and leaving the US possible as you complete humanitarian efforts. In these cases, you may file for Form I-131 yourself or have someone file on your behalf from within the United States.
If you are a green card holder and will be outside the US for less than a year, you will generally not need Form I-131. However, if you will be traveling for about one year, you may want to get Form I-131. If you will be traveling for longer than a year, you will need Form I-131 to re-enter the US. It is important to file Form I-131 before leaving the country, so that if you expect your trip may take longer than anticipated, apply for Form I-131 just in case.
There are other situations in which you may need Form I-131 as well. For example, if you are a permanent resident of the US but do not have a passport from your home country you will need to file Form I-131 before leaving the US. As well, if you are an asylum seeker or refugee you may also need Form I-131 if you plan to leave the US.