Citizenship Application Form N-400 is used for obtaining US Citizenship (naturalization). The Citizenship Form N-400 is completed by Lawful Permanent Residents (green card holders) over the age of 18 who meet the eligibility requirements to apply for Citizenship. Prepare US Citizenship Application online. Apply for US Citizenship using citizenship application form N-400 via Immigration Direct. Free eligibility quiz & instructions included. More info about at http://www.immigrationdirect.com/us-citizenship/U-S-Citizenship-application-Form-N-400.jsp