A Social Security Number (SSN) is a unique nine-digit number issued to U.S. citizens, permanent residents, and temporary residents under section 205 of the Social Security Act. The SSN is issued by the Social Security Administration (SSA) and is used to track income and determine benefits. The SSN consists 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, it represented the state in which the card was issued. Since 1972, 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. The group number has no special geographic or data significance and is used to break the numbers into blocks of convenient size for SSAs processing operations and for controlling the assignments to the states. The serial number represents a numerical series from 0001 to 9999 within each group.

The SSN is a crucial identifier used for various purposes, including obtaining credit, opening bank accounts, and filing tax returns. However, it is also susceptible to illegal use by those involved in identity theft and fraud. To get an SSN, one must file Form SS-5 with the Social Security Administration. The SSA changed the SSN assignment process to "SSN randomization" in 2011, which assigned SSNs randomly and allowed the assignment of area numbers 734–749 and 773–899. These are not Taxpayer Identification Numbers (TINs), which include additional area numbers.