"Nevermind the bullets; how much all these coffins costin' ya?"
— The Engineer on posthumous costs
Ammo, or ammunition, is a resource that contributes to a player's weapon ammunition supply. Ammunition can be found in the form of Ammo crate pickups, dropped weapons, or destroyed building parts. Ammo is universal, meaning that ammo in the form of any dropped weapon can be used in any other weapon; ammo acquired from a dropped Flamethrower will still be usable as Sniper Rifle ammo, and can also be used to fill the Cloak charge for the Spy's Invis Watch and Dead Ringer. Additionally, the amount of ammo in a pick-up is applied to all weapons equipped, not just the one currently held. Reloading cannot be performed without having reserve ammunition. Should the player expend all ammunition with a weapon, it will be automatically switched with another and will be unavailable for use until more ammo is obtained.