To apply for citizenship, the applicant must maintain residency in the U.S. as a Green Card holder for 5 years. Out of the 5 years, the applicant must be physically present in the U.S. for at least two and a half years.

For an applicant who is married to a U.S. citizen, the requirements slightly differ. The applicant can apply for citizenship after maintaining residency as a Green Card holder for 3 years. In addition, he or she must be married to the U.S. citizen for at least 3 years and must have been physically present in the U.S. for one and a half years out of the 3 years.

The procedure is straightforward. Applicants would need to submit the application form, pay the processing fees and then undergo an interview and a biometrics test. In addition, applicants would need to demonstrate their knowledge of English and the U.S. history and government by taking an English and civics test. In the end, the applicants must take an Oath of Allegiance to receive the certificate of naturalization.

For more information, please visit the government website:
http://www.uscis.gov/us-citizenship/citizenship-through-naturalization