The Eastern Front (Die Ostfront 1941-1945) of the European Theatre of World War II was a theater of war between the German Reich and the Soviet Union which encompassed central and eastern Europe from June 22, 1941 to May 9, 1945. The German term for this conflict is the 'Russlandfeldzug 1941–1945' (Russian campaign) or more commonly the 'Russian front' . Nazi propaganda dubbed the conflict The Crusade against Bolshevism. In all Soviet and the majority of Russian sources, the conflict is referred to as the Great Patriotic War, and includes operations against Japan in 1945. Some scholars of the conflict use the term Russo-German War, while others use Soviet-German War, Nazi-Soviet War, German-Soviet War, or Axis-Soviet War.

It was the largest theater of war in history and was notorious for its unprecedented ferocity, destruction, and immense loss of life. More people fought and died on the Eastern Front than in all other theaters of World War II combined. With over 20 million dead, many of them civilians, the Eastern Front has been called a war of extermination. It resulted in the destruction of the Third Reich and the partition of Germany and the rise of the Soviet Union as a military and industrial superpower.

The series of events preceding the opening of the Eastern Front included the invasion of Poland in 1939 by Nazi Germany and the resulting fourth partition of Poland when the Soviet Union used the invasion as a pretext to occupy the eastern regions of the country as outlined in the secret codicil to the August 1939 Soviet-German non-aggression pact, which also paved the way for the 1940 Soviet annexation of the Baltic States, and the annexation of Bessarabia.

This article, however, concentrates on the much larger conflict fought from June 1941 to May 1945, in which the two principal belligerent powers were Nazi Germany and the Soviet Union. The Soviet-Finnish Continuation War may be considered the northern flank of the Eastern Front.