National Insurance (NI) is a fundamental component of the welfare state in the United Kingdom. It acts as a form of social security, since payment of NI contributions establishes entitlement to certain state benefits for workers and their families.

Introduced by the National Insurance Act 1911 and expanded by the Labour government in 1948, the system has been subjected to numerous amendments in succeeding years. Initially, it was a contributory form of insurance against illness and unemployment, and eventually provided retirement pensions and other benefits.

Currently, workers pay contributions from the age of 16 years, until the age they become eligible for the State pension. Contributions are due from employed people earning at or above a threshold called the Lower Earnings Limit, the value of which is reviewed each year. Self-employed people contribute partly through a fixed weekly or monthly payment and partly on a percentage of net profits above a threshold, which is reviewed periodically. Individuals may also make voluntary contributions to fill a gap in their contributions record and thus protect their entitlement to benefits.

Contributions are collected by HM Revenue and Customs (HMRC). For employees, this is done through the PAYE (Pay As You Earn) system along with Income Tax, repayments of Student Loans and any Apprenticeship Levy which the employer is liable to pay. National Insurance contributions form a significant proportion of the UK Government's revenue, raising £145 billion in 2019-20 (representing 17.5% of all tax revenue).

The benefit component includes several contributory benefits, availability and amount of which is determined by the claimant's contribution record and circumstances. Weekly income and some lump-sum benefits are provided for participants upon death, retirement, unemployment, maternity and disability. In order to obtain the benefits which are related to the contributions, a National Insurance number is necessary.