The process of naturalization grants foreign born citizens or nationals U.S. citizenship, provided they meet the requirements Congress established. Previously, I have written about getting citizenship through military service. This information is for civilians wishing to become naturalized citizens.
The most common way for a person to become a citizen is by being a lawful permanent resident (green card holder) for at least 5 years and is at least 18 years old. After 5 years, a potential applicant may fill out “Form N-400” to start the process. During the past 5 years, the applicant must have spent at least 30 months (2.5 years) of that time inside the United States. Also, the applicant may not have had any foreign trips that lasted longer than 6 months. (These rules exist under the rationale that applicants who spends more than half their time outside the country may not have strong enough ties or allegiance to the United States.)
Another way a person can apply is if they are at least 18 years old, have been a lawful permanent resident for 3 years, and have been married to and living with a U.S. citizen for 3 years. The U.S. citizen spouse must have been a citizen for all of that 3 year period. Such applicants also fill out Form N-400.
This next way is relatively rare but is worth mentioning. A small group of people are classified as “U.S. nationals” and yet were not born U.S. citizens. People born in some U.S. territories, such as American Samoa, belong in this category. U.S. nationals who have lived in a state long enough to be legal residents can request naturalization through Form N-400; it is not required that they be a permanent resident first.
For some people it may be possible to have USCIS declare that they automatically became a citizen after they were born. This requires filling out Form N-600 “Certificate of Citizenship.” Often, the N-600 is naturalization as well, since such applicants are born as citizens of a different country but become citizens after birth. The N-600 deserves its own post, however.