Ostarbeiter was a Nazi German designation for foreign slave workers gathered from occupied Central and Eastern Europe to perform forced labor in Germany during World War II. The Germans started deporting civilians at the beginning of the war and began doing so at unprecedented levels following Operation Barbarossa in 1941. They apprehended Ostarbeiter from the newly-formed German districts of Reichskommissariat Ukraine, District of Galicia, and Reichskommissariat Ostland. These areas comprised German-occupied Poland and the conquered territories of the Soviet Union. According to Pavel Polian, although the Ostarbeiter from most occupied territories were predominantly men, of the "eastern workers" taken from occupied Soviet territories over 50% were women, and of those from Poland nearly 30% were women. Eastern workers included ethnic Ukrainians, Poles, Belarusians, Russians, Armenians, Tatars, and others. Estimates of the number of Ostarbeiter range between 3 million and 5.5 million.

The use of slave and forced labour in Nazi Germany and throughout German-occupied Europe during World War II took place on an unprecedented scale. It was a vital part of the German economic exploitation of conquered territories. It also contributed to the mass extermination of populations in occupied Europe. The Germans abducted approximately 12 million people from almost twenty European countries; about two thirds came from Central Europe and Eastern Europe. Many workers died as a result of their living conditions – extreme mistreatment, severe malnutrition and abuse were the main causes of death. Many more became civilian casualties from enemy (Allied) bombing and shelling of their workplaces throughout the war. At the peak of the program the forced labourers constituted 20% of the German work force. Counting deaths and turnover, about 15 million men and women were forced labourers at one point during the war.