ROF Thorp Arch was one of sixteen Second World War, UK government-owned Royal Ordnance Factory, which produced munitions by "filling" them. It was a medium-sized filling factory (Filling Factory No. 9). It was located on the banks of the River Wharfe, north east of the two villages of Boston Spa and Thorp Arch; and four miles south east of the town of Wetherby, West Yorkshire in England.