A national ID, or national identification card, is a portable document that is used to verify a persons identity and citizenship. It is issued by a government or state as proof of a persons identity and can contain personally identifiable information (PII) or data. The appearance, data, and functions of national ID cards can vary depending on the country, and some countries require all citizens to have and carry a national ID card, while others issue them on a voluntary basis. The information included on a national ID card generally includes the full name, date of birth, and a unique identifying number. National ID cards can also serve various functions beyond identity verification, such as a travel document within certain regions, proof of eligibility to work, or access to government services and benefits.

The controversy around national ID cards often centers around the balance between enhancing security and protecting individual privacy rights. The terrorist attacks on September 11, 2001, had a significant impact on the discourse around national identity cards, particularly in countries such as the United States, which do not have a national ID system. Some countries, like Hungary, do not have a national identification number, while others, like Bangladesh, issue a compulsory identity document to every citizen upon turning 18 years of age.