A trigger is a mechanism that actuates the firing sequence of firearms. Triggers almost universally consist of levers or buttons actuated by the index finger. Rare variations use the thumb or weak fingers to actuate the trigger.

Examples are the M2 Browning machine gun and the Springfield Armory M6 Scout respectively. Firearms use triggers to initiate the firing of a cartridge in the firing chamber of the weapon. This is accomplished by actuating a striking device through a combination of spring and kinetic energy operating through a firing pin to strike and ignite the primer.

There are two primary types of striking mechanisms, hammers and strikers. Hammers are spring-tensioned masses of metal that pivot on a pin when released and strike a firing pin to discharge a cartridge. Strikers are, essentially, spring-loaded firing pins that travel on an axis in-line with the cartridge eliminating the need for a separate hammer.

The connection between the trigger and the hammer is generally referred to as the sear surface. Variable mechanisms will have this surface directly on the trigger and hammer or have separate sears or other connecting parts. There are numerous types of trigger mechanisms.

They are categorized according to which functions the trigger is to perform. In addition to releasing the hammer or the striker, a trigger may cock the hammer or striker, rotate a revolver's cylinder, deactivate passive safeties, select between semi-automatic and full-automatic fire such as the Steyr AUG, or pre-set a 'set trigger.' Most modern firearms use the trigger to deactivate passive safeties but this does not change how they are identified.

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