Military service is service by an individual or group in an army or other militia, air forces, and naval forces, whether as a chosen job (volunteer) or as a result of an involuntary draft (conscription).

Some nations (e.g., Mexico) require a specific amount of military service from every citizen, except for special cases, such as limitation determined by a military physical or religious belief. In the United States, a mental disorder does not necessarily disqualify a recruit so long as no treatment had been given within 36 months. Most countries that use conscription systems only conscript men; a few countries also conscript women. For example, Norway, Sweden, North Korea, Israel, and Eritrea conscript both men and women. However, only Norway and Sweden have a gender-neutral conscription system, where men and women are conscripted and serve on equal formal terms. Some nations with conscription systems do not enforce them.

Nations which conscript for military service typically also rely on citizens choosing to join the armed forces as a career.

Some nations with armed forces do not conscript their personnel (e.g. most NATO and European Union states). Instead, they promote military careers to attract and select recruits; see military recruitment.

Some, usually smaller, nations have no armed forces at all or rely on an armed domestic security force (e.g. police, coast guard).

Conscription has been a topic of contention around the world.