The Medium Tank M2 was a United States Army tank that was first produced in 1939 by the Rock Island Arsenal, just prior to the commencement of the war in Europe. Production was 18 M2 tanks, and 94 slightly improved M2A1 tanks, for a total figure of 112. Events in Western Europe and on the Eastern Front rapidly demonstrated that the M2 was obsolete, and it was never used overseas in combat; it was used for training purposes throughout the war.

The M2's unique features included an unusually large number of machine guns, bullet deflector plates, and sloped armor on the hull front (glacis plate). The main armament was a 37 mm gun, with 32 mm armor; the M2A1 had a 51 mm gun mantlet. The features of the M2 series development, both good and bad, provided many lessons for U.S. tank designers that were later applied with great success in the M3 Lee, M4 Sherman and many other armored fighting vehicles.

Rock Island Arsenal started work on a new medium tank, based on the design of the M2 Light Tank. Initially designated the T5, the redesigned model (with a 350 hp R-975 radial engine) was redesignated as the M2 Medium Tank in June, 1939. After the first 18 units had been produced at Rock Island Arsenal and evaluated by the army, the upgraded M2A1 specification was approved with a redesigned turret and a more powerful engine.

Medium tank was a classification of tanks; the medium being intermediate in size and weight and armament between heavy tanks and light tanks.

The medium tank concept has been eclipsed by the main battle tank.

The first tanks to carry the name Medium appeared in the First World War with the British Medium Mark A "Whippet"

The medium tank doctrine came into use in the interwar period. Its existence outlasted the super-heavy tank and heavy tank but eventually was eclipsed by the main battle tank.

Medium tanks of the interwar period included the British Vickers Medium Mark II and the Soviet multi-turreted T-28. In the period leading up to World War II, the British stopped using the term Medium for their tanks as the new philosophy of Cruiser tank and Infantry tank which defined tanks by the role came into use.

There were medium tanks that focused on anti-infantry capabilities (such as in World War II: the Panzer IV short-barrel, and the M4 Sherman), and medium tanks which were more focused on the anti-tank role, mounting high-velocity tank guns. The French cavalry tanks (Chars de Cavalerie) focused on speed in addition to power and protection of the other designs. They were similar to what other countries called medium tanks.