United States citizenship is acquired through birth or by naturalization. You may apply for naturalization if: (i) you have been a lawful permanent resident for 5 years; (ii) you have been a lawful permanent resident for 3 years and have been married to a U.S. citizen for those 3 years; (iii) you are a lawful permanent resident child of U.S. citizen parents; or (iv) you have qualifying military service. Children under 18 years of age may automatically become citizens when their parents naturalize.