The double genocide theory is a way of looking at history which says that two different kinds of horrible things happened in the same place at the same time. In particular, the double genocide theory looks at what happened during the Holocaust in World War II. In the Holocaust, millions of Jewish people were killed by Nazis in Germany. At the same time, millions of non-Jewish people were also killed in other parts of Europe, mostly Slavic people and Roma people. The double genocide theory says that both of these terrible events were part of one big event: the genocide of European Jews and other undesirables. This means there were two kinds of genocide happening at the same time – one against the Jews and one against other people.