A Social Security number, sometimes referred to as a SSN, is an identifiable number consisting of nine digits that is issued to every citizen of the United States and to all permanent residents and temporary employed residents of the US. There are some exceptions, however, since certain religious groups are exempted from the requirement to use Social Security numbers. This means that the Social Security system is not a universal one, although it is so common that most people will need to use a Social Security number at some point in their lives.

Social Security numbers are issued by the Social Security Administration. An application must be made to this agency in order to be issued a Social Security number. Recipients will be sent a card upon which their Social Security number has been printed. The number is usually written in three chunks, of three, two and four digits, in the form "000-00-0000".

Traditionally the first three digits represent the Area Number, which indicated the geographic region in which the Social Security number was given, according to the applicant's mailing address. The second group of two digits is the Group Number and the final group of three digits is the Serial Number. Social Security number issued after June 25th 2011 will not follow this pattern. Instead, they will be produced randomly, without any geographical associations. Certain numbers are never issued, including area code 666, area, group or serial numbers that are all zeros, and certain numbers that are only ever used in adverts.

When the Social Security system first started, people typically obtained their Social Security number after the age of 14, when they began working. Today, however, it is usual for parents to apply for a Social Security number soon after their child has been born. They will then be able to use their child's Social Security number when they fill in their tax returns, in order to claim certain deductions. It is possible to apply for a Social Security number at the same time as an application for a birth certificate is completed.

The main purpose of the Social Security number is to enable the tracking of individuals so that they can be accurately taxed. However, Social Security numbers have also developed a secondary usage within the United States, as a form of national identification. Social Security numbers are often used on employee, credit, patient and student records, and they have also been used in the US military as identification numbers.

Social Security numbers are primarily used as part of the Social Security system. Taxpayers are identified by their Social Security number while they are working. As they work, they will be taxed. Some of this tax will go to the Social Security system, which is a form of social insurance. When the worker retires, they will be eligible to receive certain Social Security benefits, according to the amount that they have contributed through tax to the Social Security system during their lifetime. The Social Security number makes it possible to track contributions and to ensure that everyone receives the retirement benefits to which they are entitled.