A Social Security Number (SSN) is a unique nine-digit numerical identifier assigned to U.S. citizens, permanent residents, and eligible nonimmigrant workers in the United States. It was created in 1936 as part of the New Deal Social Security program to track individuals earnings history for Social Security benefit entitlement and computing benefit levels. The SSN is issued by the Social Security Administration (SSA), an independent agency of the United States government.

The SSN is composed of three parts: the Area Number, the Group Number, and the Serial Number. The Area Number is assigned based on the geographical region, and prior to 1972, cards were issued in local Social Security offices around the country, and the Area Number represented the state in which the card was issued. Since 1972, when the SSA began assigning SSNs and issuing cards centrally from Baltimore, the Area Number assigned has been based on the ZIP code in the mailing address provided on the application for the original Social Security card.

To get a Social Security number, one must file Form SS-5 with the Social Security Administration. The SSN is very important to have due to its direct connection to the future benefits one may be entitled to receive from the U.S. government. The SSN has become a de facto national identification number for taxation and other purposes, although its original purpose was to track individuals for Social Security benefits.

SSNs are susceptible to illegal use by those involved in identity theft and fraud. Therefore, it is important to keep ones SSN confidential and not share it with anyone unless it is necessary.