When German men were called up for military service, Nazi German authorities rounded up civilians to fill in the vacancies and to expand manufacturing operations.

Some labourers came from Germany but exponentially more from roundups in the German-occupied territories.

There were many affected populations who could be grouped by various (often overlapping) variables such as geographic, ethnic, religious, political, and health categories.

They included German political prisoners of the SA, Gestapo, and SS; foreign civilian men and women from occupied territories of Eastern Europe (Ostarbeiter); prisoners of war; institutionalized people (mentally or physically disabled people, or medical and psychiatric patients); and various ethnic, religious, or ethnoreligious groupings (for example, Jews, Sinti, Romani, Yeniche, and Jehovah's Witnesses).

Nazi concentration camps were often meant not only for forced labor but also extermination.