The Picatinny rail is a bracket used on some firearms in order to provide a standardized mounting for sniper scopes and other accessories. The standard was first published by the Picatinny Arsenal in 1913, and thus carries the official title MIL-STD-1913.

The rail is typically placed directly on the barrel, in the position normally occupied by the rear sights. Shaped in cross-section roughly like a wide T, scopes are mounted on the rail by sliding them on from one end or the other. In order to provide a stable platform, the rail should not flex as the barrel heats and cools. For this reason most Picatinny rails are cut crosswise, to give them considerable room to expand and contract lengthwise.

Originally used only for scopes, the rails were typically found only on larger caliber rifles. With the increased use of night vision scopes, they started to appear on smaller assault rifles as well, to the point today where they are replacing the original "iron sights" with versions that can be mounted on the rails.

Once the rails became fairly common, they started to be used for other accessories, flashlights for instance. This led to a sort of chain reaction, where practically every accessory is now designed to be mounted on a rail, including bipods, bayonets and laser sights. In turn, this had led to the introduction of "small rails" that can be fastened to the guns in various locations, and in some cases, entire grips with rails built-in on all sides. Short rails now appear even on shotguns and pistols.