A person may become a U.S. citizen (1) by birth or (2) through naturalization. Generally, people are born U.S. citizens if they are born in the United States or if they are born to U.S. citizens. If you are not a U.S. citizen by birth or did not acquire U.S. citizenship automatically after birth, you may still be eligible to become a citizen through the normal naturalization process. People who are 18 years and older use the "Application for Naturalization" (Form N-400) to become naturalized. Persons who acquired citizenship from parent(s) while under 18 years of age use the "Application for a Certificate of Citizenship" (Form N-600) to document their naturalization. Adopted children who acquired citizenship from parent(s) use the "Application for a Certificate of Citizenship on Behalf of an Adopted Child" (Form N-643) to document their naturalization.