Military ranks are a system of hierarchical relationships, within armed forces, police, intelligence agencies or other institutions organized along military lines. The military rank system defines dominance, authority, and responsibility in a military hierarchy. It incorporates the principles of exercising power and authority into the military chain of command—the succession of commanders superior to subordinates through which command is exercised. The military chain of command constructs an important component for organized collective action.

Uniforms denote the bearer's rank by particular insignia affixed to the uniforms on a number of countries. Ranking systems have been known for most of military history to be advantageous for military operations, in particular with regards to logistics, command, and coordination. As time went on and military operations became larger and more complex, military ranks increased and the ranking systems themselves became more complex.

Rank is not only used to designate leadership, but to establish pay-grade as well. As rank increases, pay-grade follows, but so does amount of responsibility.

Within modern armed forces, the use of ranks is almost universal. Communist states have sometimes abolished ranks (e.g., the Soviet Red Army 1918–1935, the Chinese People's Liberation Army 1965–1988, and the Albanian People's Army 1966–1991), but they had to re-establish them after encountering operational difficulties of command and control.