Operating in all weathers, invariably at night, often at low level and usually without fighter escort, Allied 'Special Duties' (SD) squadrons and units played a vital role in the Second World War. By their very nature, these operations went unreported in wartime and for considerable time thereafter, but their importance in prosecuting the war of resistance in Occupied Europe was immense. Emerging from a time of rudimentary communications and transport, dedicated air units had been established by 1939 to fly agents into enemy territory, but their aircraft were hand-me-downs and the SD was considered a poor third to fighter and bomber operations. However, the tasks undertaken by these men and their aircraft were often more hazardous and demanding than any other type of wartime flying: for every SOS, SIS or OSS agent who lost their life in the field, at least one RAF or USAAF airman was killed while flying SD operations. Here, David Oliver examines the exploits of British, Commonwealth, American, Free European, Soviet, German, Italian and Japanese airmen and units, supported by first-hand accounts and archive photographs.