The 8.8 cm Flak 18/36/37/41 (commonly called the eighty-eight) was a German 88 mm anti-aircraft and anti-tank artillery gun from World War II. It was widely used by Germany throughout the war, and was one of the most recognized German weapons of that conflict. Development of the original model led to a wide variety of guns.

The name applies to a series of related guns, the first one officially called the 8.8 cm Flak 18, the improved 8.8 cm Flak 36, and later the 8.8 cm Flak 37.^[Flak is a contraction of German Flugzeugabwehrkanone meaning "aircraft-defense cannon", the original purpose of the eighty-eight. In English, "flak" became a generic term for ground anti-aircraft fire. In informal German use, the guns were universally known as the Acht-acht ("eight-eight").

The versatile carriage allowed the eighty-eight to be fired in a limited anti-tank mode when still on its wheels; it could be completely emplaced in only two-and-a-half minutes. Its successful use as an improvised anti-tank gun led to the development of a tank gun based upon it: the 8.8 cm KwK 36, with the "KwK" abbreviation standing for Kampfwagen-Kanone (literally "battle vehicle cannon", or "main battle tank cannon"), meant to be placed in a gun turret as the tank's primary armament. This gun served as the main armament of the Tiger I heavy tank.

In addition to these Krupp designs, Rheinmetall later created a more powerful anti-aircraft gun, the 8.8 cm Flak 41, which was produced in relatively small numbers. Krupp responded with another prototype of the long-barreled 88 mm gun, which was further developed into the anti-tank and tank destroyer 8.8 cm PaK 43 gun used for the Elefant and Jagdpanther, and turret-mounted 8.8 cm KwK 43 heavy tank gun of the Tiger II.