The most common way that people become U.S. citizens is by being born in the United States. However, a few people who were born outside the United States are also considered U.S. citizens by virtue of one or both of their parents being U.S. citizens (either native-born or naturalized). In technical terms, the person has either "acquired" or "derived" U.S. citizenship.
Such people will not have a U.S. birth certificate, however. In some cases, their parents may have gone to a U.S. consulate in the country where they were staying and obtained confirmation of their birth and U.S. citizenship, but not always. So, in order to prove their U.S. citizenship, such people can file for a Certificate of Citizenship.
If the application is successful, the U.S. government will issue the certificate, and number it either N-560 or N-561. Using that certificate, the applicant will have an easy time obtaining a U.S. passport, Social Security Number, and more.
Note: Do not confuse this process with the one used by U.S. lawful permanent residents (green card holders) who have completed the required number of years in the U.S. (usually five) and are ready to apply for naturalization. This is a separate path to U.S. citizenship, requiring a different form (N-400) and a different process, which includes an interview and a test of the immigrant's English language abilities and knowledge of U.S. history and government. If successful, such applicants will be issued a Certificate of Naturalization, which is different than the certificate described in this article. If an applicant seeking naturalization submits the forms described below, requesting a Certificate of Citizenship instead of naturalization, he or she will be denied.
If you want to request a Certificate of U.S. Citizenship, you have to fill out either Form N-600 or N-600K, depending on whether you are filing on your own behalf or on behalf of a child whom you are adopting from another country. These forms, along with instructions, the latest fee, and other filing information are available on the Forms page of the USCIS website.
If you need assistance figuring out whether you qualify for U.S. citizenship based on the citizenship of your parent or parents, or in filing your forms or applying for a certificate of U.S. citizenship, contact an experienced US immigration attorney.