A co-production between the United Kingdom and the United States, "The Devils" was partly adapted from the 1952 non-fiction book "The Devils of Loudun" by Aldous Huxley, and partly from the 1960 play "The Devils" by John Whiting, also based on Huxley's book. United Artists originally pitched the idea to Russell but abandoned the project after reading his finished screenplay, as they felt it was too controversial in nature. Warner Bros. subsequently agreed to produce and distribute the film. The majority of principal photography took place at Pinewood Studios in late 1970.