"The Master Key System" is a personal development book by Charles F. Haanel (1866-1949) that was originally published as a 24-week correspondence course in 1912, and then in book form in 1916. The ideas it describes and explains come mostly from New Thought philosophy. It was one of the main sources of inspiration for Rhonda Byrne's film and book "The Secret" (2006).

The book describes many "New Thought beliefs" such as the law of attraction, creative visualization and man's unity with God, and teaches the importance of truth, harmonious thinking and the ability to concentrate.

By 1933 it had allegedly sold over 200,000 copies worldwide. Haanel practiced the financial principles he preached and was a self-made success who owned several major companies. According to Walter B. Stevens, the author of In St. Louis: History of the Fourth City, "He (Mr. Haanel) was president of the Continental Commercial Company, president of the Sacramento Valley Improvement Company, and president of the Mexico Gold & Silver Mining Company."
About the Author: Charles Francis Haanel (May 22, 1866 - November 27, 1949) was a noted American New Thought author, philosopher and a businessman. He is best known for his contributions to the New Thought Movement through his book The Master Key System.

The Haanel family was of Swedish extraction but had lived in Silesia, Prussia, before emigrating to Canada and thence to the United States.

In St. Louis: History of the Fourth City, the author Walter B. Stevens wrote that "Charles F. Haanel was born in Ann Arbor, Michigan, the son of Hugo and Emeline (Fox) Haanel." He was the fourth of six children.[6] According to Stevens, "The family moved to St. Louis, Missouri when Charles was a child. His first job was as an office boy for the National Enameling & Stamping Company in St. Louis, and he worked for this firm in varying capacities for fifteen years before striking out on his own as a writer and businessman."

In 1885 he married Esther M. Smith. They had one son and two daughters. In 1891 his wife died. In 1908 he married for the second time, to Margaret Nicholson of St. Louis, whose father was W. A. Nicholson.

Haanel's book The Master Key System was published in 1912 when he was 46 years old. It is written in the form of a course in New Thought, mental development, financial success, and personal health. The book was heavily promoted in the pages of Elizabeth Towne's New Thought magazine The Nautilus. By 1933 it had allegedly sold over 200,000 copies worldwide. Haanel practiced the financial principles he preached and was a self-made success who owned several major companies. According to Stevens, writing in 1909, "He was president of the Continental Commercial Company, president of the Sacramento Valley Improvement Company, and president of the Mexico Gold & Silver Mining Company."