U.S. citizenship gives a person as many rights as the U.S. has to offer; for example, the right to vote, petition for family members to immigrate, and live abroad without losing the right to return. For these reasons, citizenship is not easily obtained.
To become a U.S. citizen through the process known as naturalization, you must first have a green card (permanent residence) and then meet other requirements, listed below. There are only a few rare exceptions in which a person goes straight from having no U.S. status to getting U.S. citizenship; some are discussed in U.S. Citizenship by Birth or Through Parents.
If you are interested in applying for U.S. citizenship, first make sure that all of the following apply to you:
Applying for citizenship opens your whole immigration history to review. U.S. Citizenship and Immigration Services (USCIS) will carefully investigate your background. If it discovers something wrong—for example, that you used fraud to get your green card or abandoned your residency by making your home outside the United States—it can strip you of your green card and send you out of the country.
You'll need to complete a citizenship application on USCIS Form N-400 (see Filling Out USCIS Form N-400) and send it in with a copy of your green card, the required photos, and the appropriate fee. After filing your application, you will probably wait for many months, depending on your local USCIS office. First, you will be called in for a fingerprint (biometrics) appointment, and later an interview appointment.
At the interview, a USCIS officer will test your English language ability (unless you fit within an exception) and your knowledge of U.S. history and government (though with a shorter list of possible question if you are 65 or older and have been a permanent resident of the U.S. for at least 20 years).
Applicants who are disabled can ask for accommodations at the interview, such as a sign language interpreter or wheelchair accessibility.
If all goes well at the interview, you'll receive an appointment for your swearing-in (oath) ceremony. At that time, you actually become a citizen, and receive a certificate of naturalization to prove it.
As a citizen, you can petition to have close family members join you in the United States. For details, see Green Cards for Your Family: Sponsorship Rules.
For more on the eligibility and application requirements for citizenship, including important exceptions, the rights of disabled persons, and the details of how to apply, see, Becoming a U.S. Citizen: A Guide to the Law, Exam & Interview, by Ilona Bray (Nolo).