The Thompson is a sub machine gun of US origin that was developed in 1919. The Thompson got its name from its inventor John T. Thompson, but is also known under various nicknames including Tommy Gun, Trench Broom and Chicago Typewriter. The original version with forward pistol grip and 50 round drum magazine has become an iconic firearm associated with gangsters in the US.

The Thompson is a fully automatic sub machine gun made out of metal with the forearm, pistol grip and stock made out of wood. At the time of development the Thompson was a finely made weapon produced to high standards, resulting in a high price tag. The earlier models used a delayed blowback operation based on the Blish principle. Later military versions use the much simpler and cheaper simple blowback operation.

The Thompson is a select-fire weapon. It fires the large and powerful .45 ACP round at a high rate of fire from 20 or 30 round magazines or 50 round drums. Versions developed for the military featured a lower rate of fire. Targets out to 200 meters can be engaged, although with automatic fire the effective range is 50 meters.

The Thompson was widely used by US law enforcement and criminals during the 1930's and just before World War 2 it was adopted by the US armed forces. The Thompson was well liked for its quality and firepower. Although less costly versions of the Thompson reduced the costs from over 200 dollars to as little as 45 dollars the Thompson was replaced in US service by the even cheaper M3 Grease Gun.