Hopalong Cassidy is a fictional cowboy hero created in 1904 by the author Clarence E. Mulford, who wrote a series of popular short stories and twenty-eight novels based on the character.
In his early writings, Mulford portrayed the character as rude, dangerous, and rough-talking. Beginning in 1935, the character—as played by movie actor William Boyd in films adapted from Mulford's books—was transformed into a clean-cut on-screen hero. A total of sixty-six immensely popular films were released, only a few of which relied on Mulford's original story lines. Mulford later revised and republished his earlier works to be more consistent with the character's new, polished on-screen persona.