The monograph is devoted to a problem that has been poorly studied in Russian historiography—collaborationism on the territory of Karelia during the Second World War. The author examines the facts of cooperation of the Finnish civilian population and prisoners of war with the Soviet authorities during the Winter War (1939–1940) and the cooperation of Soviet people and prisoners of war of the Red Army with the Finnish occupation administration during the occupation of the territory of Soviet Karelia (1941–1944). The book was prepared on the basis a wide range of sources, the basis of which was archival documents from the funds of state and departmental archives of Russia and Finland. Many archival documents are being introduced into scientific circulation for the first time.