The Holocaust was the genocide of approximately six million European Jews and millions of others during World War II by Nazi Germany. Jews were confined to overcrowded ghettos and transported to concentration camps, where most were killed in gas chambers or by exhaustion or disease. Medical experiments were also conducted on human subjects in the camps. Romani people were also targeted and killed in large numbers, though records of their treatment are less complete. Political opponents and minority groups were also imprisoned and killed in the camps.