American citizenship can be acquired by birth or through naturalization. Naturalization is the process by which non-citizens become American citizens.
If you were born in the United States, or one or both of your parents were American citizens at the time of your birth, you may already be a citizen of the United States.
If you believe you are already a U.S. citizen, you can obtain proof of your citizenship by filing Form N-600 with USCIS for a Certificate of Citizenship.
If you are not a United States citizen but you are eligible and would like to become one you will need to file Form N-400, Application for Naturalization. 
If you would like to become a citizen of the United States you should contact an immigration attorney to help ensure that every stage and aspect of the application process will be taken care of responsibly and effectively.
Eligibility for Naturalization
Generally you will need to become a legal permanent resident in the United States before you can become a citizen.
Most people are required to remain in the United States as a legal permanent resident for several years before becoming a citizen. 
If you are at least eighteen years old and have been a legal permanent resident for at least five years, you are likely eligible for citizenship as long as there are no special circumstances such as criminal convictions on your record.
If you are at least eighteen years old and married to a U.S. citizen then you are likely eligible for naturalization as long as you meet each of the following requirements:
- You are currently married to and living with your citizen spouse
- You and your spouse have been married and lived in the United States for at least three years
- Your spouse has been a citizen for at least three years
- You have been a legal permanent resident in the U.S. for at least three years
If you served as a member of the United States armed forces during any of the major conflicts involving American troops (including the wars in Korea, Vietnam, the Persian Gulf, or military action on or after September 11, 2001), you are not required to be a permanent resident in order to be naturalized.
If you were married to a U.S. citizen who died during a period of honorable active duty service in the U.S. military, you must simply be a permanent resident at the time of your naturalization interview.
If any of the following circumstances apply to you, your eligibility for citizenship may be denied or delayed.
These exceptions include:
- You were convicted of a crime by a court in the United States
- You left the United States for one year or longer during your time as a permanent resident
In addition to the duration of legal permanent residence requirement, in order to become a citizen of the United States you will need to demonstrate the following characteristics which will be discussed later in this article:
- Good moral character
- Knowledge of English and civics
- Attachment to the United States Constitution
Application for Naturalization (Form N-400)
The first stage of the naturalization process will be for you to file your application with USCIS.
Filing your application can be time-consuming and tedious. It’s also imperative that you do everything at this stage of the process correctly.
In addition to filling out Form N-400, you will be required to provide:
- A photocopy of both sides of your Permanent Resident Card (commonly known as a Green Card)
- Two passport-style photographs of yourself including your name and Alien Registration Number written lightly in pencil on the back of each photo
- A check or money order for the application fee and biometrics services fee for fingerprinting
Depending on your circumstances, you may need to file other documents in addition to these to complete your application.
For most documents copies will be accepted but if USCIS requires original documents they will notify you.
Applications and other materials required should be mailed to the USCIS lockbox that corresponds with your state of residence.
You can check which location your state is assigned to and the address where you should send your application by visiting the USCIS website here.
Fingerprinting and Background Check
After filing your application, you will receive a letter from USCIS telling you when and where to have your fingerprints taken.
Usually, you will be told to go to an Application Support Center nearest you.
When you go to the fingerprinting location you should bring your USCIS notice letter, your Permanent Resident Card, and another form of identification with your photograph on it.
The fingerprinting process should not take long and all locations take fingerprints electronically.
USCIS will send your fingerprints to the FBI for a background check.
In some cases the prints will not be adequate and you may be contacted to return to the fingerprinting location for a second attempt.
If the FBI rejects your prints a second time you will be asked to provide more information so a background check can be performed without fingerprints.
Waiting for Your Interview Date
While USCIS processes your application, you may be asked to provide further documentation.
You will need to provide any requested documents before USCIS can schedule your interview date.
Once your FBI background check is successful and your application has been approved, you will receive a notice from USCIS with the date, time, and location for your interview.
One of the final steps for becoming a citizen of the United States will be to attend an interview with a USCIS officer.
You should arrive at your assigned location about thirty minutes early.
You will be allowed to have a lawyer present with you at the interview if you choose to do so.
Be sure to attend your interview at the assigned location and appropriate time.
If you miss your interview date, it will be possible to reschedule but this could add months to your naturalization process.
If you fail to contact USCIS within one year after missing your interview date your application will be denied.
If you know you will be unable to attend your interview at the assigned time, write to the office where you are scheduled to appear and explain why you will be unable to attend.
You will be notified by mail when a new date has been set for your interview.
In some cases, USCIS will ask you to bring certain documents to your interview.
If you fail to do so your application could be closed or denied.
If you have any questions at any time you should contact an immigration attorney for assistance.
At your interview, a USCIS officer will ask you about the following:
- Your background and evidence supporting your application
- Where you live and the length of your residence in the United States
- Your character
- Your attachment to the Constitution of the United States
- Your willingness to take an Oath of Allegiance to the United States
At your interview you will be tested on your ability to read, write, and speak English, as well as your knowledge of U.S. history and government.
You can access study materials for these exams on the USCIS website.
To test your ability to read English you will be asked to read one sentence in a manner which leads the USCIS officer to believe you understand its meaning.
To test your ability to write English you must write one sentence in a manner which the USCIS officer is able to understand.
Your ability to speak English will be determined by your answers to the questions asked in your interview.
There are one hundred civics questions which can be asked and you will be required to answer six out of ten of them correctly.
Receive the USCIS Officer’s Decision
You may receive your decision immediately at the end of your interview but often you will receive a notice in the mail several weeks later.
Your application may also be continued if you fail one of the exams.
If your application is denied you will receive notice in the mail with the reason(s) why your application was denied.
If your application is granted immediately after your interview, you may be able to take the Oath of Allegiance that same day.
Otherwise you will be provided with instructions for your ceremony date by mail with Form N-445, Notice of Naturalization Oath Ceremony.
Oath of Allegiance
If USCIS approves your application for naturalization you will be required to attend a ceremony where you must take the Oath of Allegiance.
If you are unable to attend the ceremony on the date assigned, you should return Form N-445 to USCIS with a letter explaining why you cannot attend and asking for another date.
You should arrive for the ceremony about thirty minutes early and check in when you arrive.
You will be required to turn in your Permanent Resident Card which you will no longer need because at your ceremony you will be given a Certificate of Naturalization at the ceremony.
If your ceremony is more than one day after your interview you will be asked a few questions about what you have done since the interview.
These questions can be found on the back of your Notice of Naturalization Oath Ceremony and include whether you have traveled outside the United States since your interview and whether you have claimed an exemption from military service.
The Oath which you will be required to recite can be found on the USCIS website.
Under certain circumstances including religious observance and physical disabilities you may be allowed to recite a modified version of the Oath.
After you take the Oath of Allegiance you will be given your Certificate of Naturalization and you will be entitled to all the benefits of American citizenship.
 What Should I Expect from the Naturalization Process? U.S. Citizenship and Immigration Services
 A Guide to Naturalization, U.S. Citizenship and Immigration Services