The Pak 36 (Panzerabwehrkanone 36) was a German anti-tank gun that fired a 3.7 cm calibre shell. It was the main anti-tank weapon of Wehrmacht infantry units until 1942. It was followed in this role by 5 cm Pak 38 gun.
The Pak 36, being a small-calibre weapon, was outdated by the May 1940 Western Campaign, and crews found them inadequate against allied tanks like the British Mk.II Matilda, and the French Char B1 and Somua S35. Still, the gun was effective against the most common light tanks, such as the Renault FT-17 during the Battle of France and the T-26 during Operation Barbarossa. The widespread introduction of medium tanks quickly erased the gun's effectiveness; miserable performance against the T-34 on the Eastern Front led to the Pak 36 being derisively dubbed the "Door Knocker" (Heeresanklopfgerät, literally "army door-knocking device") for its inability to do anything other than advertise its presence to a T-34 by futilely bouncing rounds off its armor.
The Pak 36 began to be replaced by the new 5 cm Pak 38 in mid 1940. The addition of tungsten-core shells (Pzgr. 40) added slightly to the armour penetration of the Pak 36. Despite its continued impotence against the T-34, it remained the standard anti-tank weapon for many units until 1942. It was discovered that Pak 36 crews could still achieve kills on T-34s, but the feat required tungsten-cored armour piercing ammunition and a direct shot to the rear or side armor from point-blank range.
As the Pak 36 was gradually replaced, many were removed from their carriages and added to SdKfz 251 halftracks to be used as light anti-armour support. The guns were also passed off to the forces of Germany's allies fighting on the Eastern Front, such as the 3rd and 4th Romanian Army. This proved particularly disastrous during the Soviet encirclement (Operation Uranus) at the Battle of Stalingrad when these Romanian forces were targeted to bear the main Soviet armored thrust. The Pak 36 also served with the armies of Finland (notably during the defense of Suomussalmi), Hungary, and Slovakia.