A national identification number is a unique identifier assigned by a government to its citizens, permanent residents, and temporary residents for the purposes of tracking them for work, taxation, government benefits, health care, and other governmentally-related functions

. The ways in which such a system is implemented vary among countries, but in most cases, citizens are issued an identification number upon reaching legal age or when they are born

. Non-citizens may be issued such numbers when they enter the country or when granted a temporary or permanent residence permit

. The format and length of the number also vary by country. For example, in Estonia, Latvia, and Lithuania, the national identification number is an 11-digit number assigned by the state and used for identification purposes

. In the United States, there is no official "national identity card," but identity documents are typically the regional state-issued driver's license or identity card, while also the Social Security card (or just the Social Security number) and the United States Passport Card may serve as national identification