The IS tanks were a series of heavy tanks developed as a successor to the KV-series by the Soviet Union during World War II. The IS acronym is the anglicized initialism of Joseph Stalin. The heavy tanks were designed as a response to the capture of a German Tiger I in 1943. They were mainly designed as breakthrough tanks, firing a heavy high-explosive shell that was useful against entrenchments and bunkers. The IS-2 went into service in April 1944 and was used as a spearhead by the Red Army in the final stage of the Battle of Berlin. The IS-3 served on the Chinese-Soviet border, the Hungarian Revolution, the Prague Spring and on both sides of the Six-Day War. The series eventually culminated in the T-10 heavy tank.

Heavy tank is a term used to define a class of tanks produced from World War I to the end of the Cold War. These tanks generally sacrificed mobility and maneuverability for better armour protection and equal or greater firepower than tanks of lighter classes.