Shortly after they came to power in 1933, the Nazis established a system of concentration camps where their political opponents and others the Nazis considered "undesirable" were imprisoned. The first groups to be detained were Communists and Jews, but later Gypsies, homosexuals, Jehovah's Witnesses, the mentally ill, and many others were also forced into the camps. After the war began, persons from conquered territories, especially those from ethnic groups the Nazis claimed were "inferior," were imprisoned.

Conditions in the camps were brutal, and millions perished from disease, malnutrition, and overwork. Inmates were used as slaves for war industries, and were subjected to sadistic torture and barbaric medical experiments.

Beginning in 1942, the Nazis implemented what they called the "Final Solution to the Jewish Question"—an attempt to kill every Jew in Europe. Jews from all parts of Europe that were under Nazi control were rounded up and killed in mass executions or sent to concentration camps where thousands were killed each day in gas chambers. This mass slaughter has come to be known as the Holocaust.

The most infamous of the concentration camps were Buchenwald, Dachau, and Ravensbrück, in Germany, and Auschwitz-Birkenau, Treblinka, and Chelmno, in Poland. Although the exact number will never be known, it is thought that more than 11 million persons were killed in concentration camps, and many more died from disease and starvation. At least 6 million Jews were killed by the Nazis, most in the camps. Although rumors about the existence of the camps leaked out during the war, few believed that something so horrible could be true until Allied troops discovered the camps in the spring of 1945.