This study makes extensive use of information drawn from internal, classified CIA files- from the records of the directors of central intelligence and of the staffs that assisted them in their role as leaders of the US Intelligence Community; from interviews conducted as part of CIA's oral history program; from organizational histories and biographies of directors of central intelligence; and from Studies in Intelligence, a journal published by CIA since 1955. With some exceptions, these sources are not individually cited in the footnotes. These internal, classified resources supplement openly available material, such as declassified official histories (Troy, Darling, Montague, and Jackson) covering William Donovan and the first five DCIs, as well as a number of memoirs, biographies, books, and commission studies devoted to intelligence, all of which are listed in the bibliography. The bibliography also lists the interviews conducted by the author for this study.