A breechloader is a firearm in which the user loads the ammunition (cartridge or shell) via the rear (breech) end of its barrel, as opposed to a muzzleloader, which loads ammunition via the front (muzzle).

Modern firearms are generally breech-loading, while early firearms before the mid-19th century were almost entirely muzzle-loading. Mortars and the Russian GP-25 grenade launcher are the only muzzleloaders remaining in frequent modern usage. However, referring to a weapon specifically as breech loading is mostly limited to single-shot or otherwise non-repeating firearms, such as double-barreled shotguns and double rifles.

Breech-loading provides the advantage of reduced reloading time, because it is far quicker to load the projectile and propellant into the chamber of a gun/cannon than to reach all the way over to the front end to load ammunition and then push them back down a long tube – especially when the projectile fits tightly and the tube has spiral ridges from rifling. In field artillery, the advantages were similar – crews no longer had to get in front of the gun and pack ammunition in the barrel with a ramrod, and the shot could now tightly fit the bore, increasing accuracy. It also made it easier to load a previously fired weapon with a fouled barrel. Gun turrets and emplacements for breechloaders can be smaller, since crews don't need to retract the gun for frontal loading. Unloading a breechloader is much easier as well, as the load can be extracted from the breech end and is often doable by hand; unloading muzzle loaders requires drilling into the projectile to drag it out through whole length of the barrel, and in some cases they are simply fired to unload.

After breech-loading became common, it also became common practice to fit recoil systems onto field guns to prevent the recoil from rolling the carriage back with every shot and ruining the aim. That provided faster firing times, but is not directly related to whether the gun is breech-loading or not. Now that guns were able to fire without the entire carriage recoiling, the crew was able to remain grouped closely around the gun, ready to load and put final touches on the aim, prior to firing the next shot. That led to the development of an armored shield fitted to the carriage of the gun, to help shield the crew from long range area or sniper fire from the new, high-velocity, long-range rifles, or even machine guns.