In the United Kingdom, National Insurance (NI) is a system of social security contributions paid by employees, employers, and self-employed individuals to fund various social welfare programs, including the National Health Service (NHS), state pensions, and other benefits.

National Insurance contributions are based on earnings and are collected by HM Revenue and Customs (HMRC). The amount of National Insurance that an individual pays is determined by their earnings, and is calculated as a percentage of their gross pay.

For employees, there are two classes of National Insurance contributions: Class 1 and Class 2. Class 1 contributions are deducted automatically from an employee's pay by their employer, and are based on their earnings above a certain threshold. Class 2 contributions are paid by self-employed individuals who earn above a certain threshold, and are calculated based on their profits.

Employers are also required to pay National Insurance contributions on behalf of their employees, based on their employees' earnings.

In addition to Class 1 and Class 2 contributions, there are other classes of National Insurance contributions that may apply to certain individuals, such as Class 3 contributions for voluntary contributions and Class 4 contributions for self-employed individuals with profits above a certain threshold.

The National Insurance system is separate from the income tax system in the UK, although National Insurance contributions are deducted alongside income tax from an employee's pay. The rates and thresholds for National Insurance contributions are reviewed each year, and any changes are usually announced in the annual Budget statement by the Chancellor of the Exchequer.