The Master Key System is a personal development book by Charles F. Haanel that was originally published as a correspondence course in 1912. Though originally a 24 week correspondence course, it was published in book form in 1916. It was first published as a book in 1916 by Psychology Publishing, St. Louis and The Master Key Institute, New York.
Along with "The Science of Getting Rich", by Wallace D. Wattles, the Master Key System is the source of Rhonda Byrne's inspiration for the book and the film "The Secret" (2006).[1][2]
Charles F. Haanel was an American author, millionaire, entrepreneur, and businessman who belonged to several Masonic societies, the American Scientific League, The Author’s League of America, The American Society of Psychical Research, the St. Louis Humane Society, and the St. Louis Chamber of Commerce.

The Master Key System has gained popularity in recent years, in no small part from its influence on the film The Secret, but also in part from a number of other rumors. The two most prominent are that it was the source of Bill Gates' inspiration and success behind Microsoft, and that the book was banned by the Roman Catholic Church. It states that Gates discovered the book as a student at Harvard. He was so inspired he dropped out and went on to found Microsoft.[3] Several sources refer to this story, however there remains no verified reference that this is anything but a rumor.[4]
Similarly, there are accounts that the book dropped out of sight partly as a result of being banned by the Church.[5] This would presumably be the Roman Catholic Church. However, again there is no verifiable reference to this being the case. An ancillary rumor to the banned by the Church rumor, is that Haanel himself wanted to keep the information in the book esoteric, and accessible only to a few. However, this defies the facts, primarily that he advertised his correspondence course in the popular media of the time (i.e., periodical advertising), and developed and distributed promotional pamphlets.