Ammunition (informally ammo) is the material fired, scattered, dropped or detonated from any weapon or weapon system. Ammunition is both expendable weapons (e.g., bombs, missiles, grenades, land mines) and the component parts of other weapons that create the effect on a target (e.g., bullets and warheads).

The purpose of ammunition is to project a force against a selected target to have an effect (usually, but not always, lethal). An example of ammunition is the firearm cartridge, which includes all components required to deliver the weapon effect in a single package. Until the 20th century, black powder was the most common explosive used but has now been replaced in nearly all cases by modern compounds.

Ammo comes in a great range of sizes and types and is often designed to work only in specific weapons systems. However, there are internationally recognized standards for certain ammo types (e.g., 5.56×45mm NATO) that enable their use across different weapons and by different users. There are also specific types of ammo that are designed to have a specialized effect on a target, such as armor-piercing shells and tracer ammo, used only in certain circumstances. Ammunition is commonly labeled or colored in a specific manner to assist in the identification and to prevent the wrong ammunition types from being used accidentally or inappropriately.