A Social Security number (SSN) is a nine-digit number issued to U.S. citizens, permanent residents, and temporary (working) residents under the Social Security Act, codified as 42 U.S.C. ﾧ 405 et seq. The number is issued to an individual by the Social Security Administration, an independent agency of the United States government and is intended to track individuals for taxation purposes. In recent years the SSN has become a de facto national identification number.