A National Insurance (NI) number is a unique personal identification number that is used in the United Kingdom to record an individual's entitlement to state benefits and to keep track of their National Insurance contributions.

NI numbers are assigned to individuals when they start work or when they claim benefits in the UK. The number consists of two letters, six numbers, and one final letter, and it remains the same throughout an individual's lifetime.

NI numbers are used by the Department for Work and Pensions (DWP) and HM Revenue and Customs (HMRC) to identify individuals and to record their National Insurance contributions. National Insurance contributions are paid by employees, self-employed individuals, and employers, and they are used to fund state benefits such as the State Pension, unemployment benefits, and healthcare. If you are working or claiming benefits in the UK, you will need to have an NI number. If you do not have an NI number, you can apply for one by contacting the DWP or HMRC. You will need to provide proof of your identity and your immigration status, and you may be required to attend an interview as part of the application process.