Form N-400 is an application for naturalization, and is used to apply for U.S. citizenship. The form is filed with the U.S. Citizenship and Immigration Service.
Form N-400 should be filed by U.S. permanent residents or U.S. military veterans who wish to become citizens of the United States, according to the U.S. Citizenship and Immigration Service. The filing fee for the form is $595 plus a biometric fee of $85 for a total fee $680, which is payable to the Department of Homeland Security. The U.S. Citizenship and Immigration Service allows forms to be filed by the applicant or by an attorney or authorized representative.