The monograph is devoted to a poorly studied problem in the domestic historiography of the Second World War. The author shows the struggle of the security forces of Karelia against the Finnish special services in the Soviet-Finnish war of 1939–1940, in the interwar period (March 1940 - June 1941) and during the Great Patriotic War on the Karelian front (June 1941 - September 1944) . Particular attention is paid to countering the activities of Finnish reconnaissance and sabotage schools in the Soviet rear in 1942–1944, located both in the occupied territory of Karelia and in the border regions of Finland. The publication was prepared on the basis of a wide range of sources, the basis of which was archival documents from state funds and departmental archives of Russia and Finland. Many archival documents are being introduced into scientific circulation for the first time.