The PaK 36 (Panzerabwehrkanone 36) was a German anti-tank gun that fired a 3.7cm caliber shell. It was developed in 1936 by Rheinmetall and first appeared in combat that year during the Spanish Civil War. It formed the basis for many other nations' anti-tank guns during the first years of World War II. The KwK 36 L45 was the same gun but was used as the main armament on several tanks, most notably the early models of the Panzer III.

The PaK 36, being a small-calibre weapon, was outdated by the May 1940 Western Campaign, and crews found them all but useless against heavy allied tanks like the British Mk.II Matilda and the French Char B1 and Somua S-35. A group of these guns claimed to have knocked out a Char by firing at its flank, but this is most likely made up. The poor performance against heavy enemy armour resulted in the PaK 36 being dubbed the "Door Knocker".

The PaK 36 began to be replaced by the new 5cm PaK 38 in mid 1940. The addition of tungsten cored shells added slightly to the armour penetration of the PaK 36. When the German troops engaged the Soviet T-34 for the first time, the PaK 36 was proven totally obsolete. Despite this, it remained the standard anti-tank weapon for many units until 1942. PaK 36 crews could still achieve kills on enemy tanks but had to wait for an opportunity to hit the tank's rear armour from close range, a task requiring nerves of steel and allowing for no second attempt.

As the PaK 36's were gradually replaced, many were removed from their carriages and added to Halftracks to be used as light anti-armour support. A number of PaK 36s were also supplied to Germany's allies. The PaK 36 served with the armies of Finland, Hungary, Romania and Slovakia. In 1943, the introduction of the Stielgranate 41 shaped charge meant that the PaK 36 could now penetrate any armour, but only at a range of less than 300 meters. The PaK 36s, together with the new shaped charges, were issued to Fallschirmjäger and other light troops. The gun's light weight meant that it could be easily moved by hand, and this mobility made it ideal for their purpose..