A National Insurance (NI) number is a personal reference number used to track tax and social security contributions in the United Kingdom. For expats, an NI number is essential if they plan to work, pay taxes, or access certain state benefits. Expats in the UK usually apply for an NI number after arriving, either as part of a visa application or separately once employment begins. Employers use the NI number to ensure the correct amount of income tax and National Insurance contributions are deducted from salary. Expats in the UK can legally start work without an NI number, but they must apply as soon as possible to avoid issues with payroll and taxation. The NI number stays with an individual for life, even if they leave and later return to the UK. Having one helps expats integrate into the tax system and ensures their employment and contribution records are accurate.