The grimmest part of the World War II was the Holocaust. Following the war, a number of histories, memoirs, and specialized studies about the Holocaust were published. One element that was largely missing, however, was the records of the various Western intelligence agencies, such as the American Office of Strategic Services (OSS) and Britain's M.I.6. Also absent was the intelligence gathers by wartime Allies communications intelligence (COMINT) agencies. Beginning in the mid-1970's, scholars of the Holocaust who had wanted to utilize the archived material of the wartime code-breaking agencies focused their research on records at the U.S. National Archives and Records Administration and the United Kingdom's Public Record Office. It was not until a number of further significant releases of information and wartime records that the amount of information made available to researchers of the Holocaust dramatically increased. This guide focuses on the two major COMINT agencies that produced intelligence about the war the British GC&CS and the U.S. Army's SIS.