If you are not a US citizen by birth or did not acquire citizenship automatically after birth, you may still be eligible to become a citizen through the normal naturalization process. If you are are 18 years and older, you use the " Application for Naturalization" (Form N?400) to become naturalized. If you acquired citizenship from parent(s) while under 18 years of age use the "Application for a Certificate of Citizenship" (Form N?600) to document your naturalization. If you are an adopted child who acquired citizenship from your parent(s) use the "Application for a Certificate of Citizenship on Behalf of an Adopted Child" (Form N?643) to document your naturalization.