National service is the system of compulsory or voluntary government service, usually military service. Conscription is mandatory national service. The term national service comes from the United Kingdom's National Service (Armed Forces) Act 1939.

The length and nature of national service depends on the country in question. In some instances, national service is compulsory, and citizens living abroad can be called back to their country of origin to complete it. In other cases, national service is voluntary.

Many young people spend one or more years in such programmes. Compulsory military service typically requires all citizens to enroll for one or two years, usually at age 18 (later for university-level students). Most conscripting countries conscript only men, but Norway, Sweden, Israel, Eritrea, Morocco and North Korea conscript both men and women.

Voluntary national service may require only three months of basic military training. The US equivalent is Selective Service. In the United States, voluntary enrollments at the Peace Corps and AmeriCorps are also known as national service.