There are two basic ways to become a United States citizen (USC): naturalization and acquisition at birth.

Naturalization
Legal permanent residents (LPRs) may qualify to become US citizens if they meet the following criteria:

1. Be 18 years or older
2. Have 5 years ( or 3 years if married to a USC) of continuous residence as an LPR (no absences from the country more than one year, and preferably none more than 6 months)
3. Have been a resident for at least 3 months in the state in which the petition is filed
4. Have been physically present for at least one half of the 5 years of continuous residence (or 3 if married to a USC)
5. Have had good moral character for the 5 (or 3) years of residence
6. Read, write and understand English at an elementary level
7. Know the fundamentals of US history and government
8. Support the US Constitution
9. Be willing to bear arms or perform work of national importance on behalf of the United States when required by law

There are exceptions to the residence and physical presence requirements for people married to overseas workers under section 319(b).

LPR children under 18 will naturalize automatically with their parent if they are in their parent's custody in the US.

Birth
All children born in the United States are US citizens.

A person born outside the US to a USC parent may acquire US citizenship at birth. Acquisition depends on the law that was applicable at the time of the person's birth, on the marital status of the parents, and on other factors such as the time the USC parent has lived in the United States.