The Replacement Army was part of the Imperial German Army during World War I and part of the Wehrmacht during World War II. It was based within Germany proper and included command and administrative units as well as training and guard troops. Its primary role was to provide replacements for the combat divisions of the regular army.

The military districts, also known in some English-language publications by their German name as Wehrkreise, were administrative territorial units in Nazi Germany before and during World War II. The task of military districts was the organization and the handling of reinforcements and resupplies for local military units. The Replacement Army (Ersatzheer) managed the districts. Responsibilities such as training, conscription, supply, and equipment were entrusted to the Ersatzheer.