David Harold Yost is an American actor and producer. He is best known for portraying Billy Cranston in Mighty Morphin Power Rangers, Mighty Morphin Power Rangers: The Movie, Mighty Morphin Alien Rangers, Power Rangers Zeo, Mighty Morphin Power Rangers: Once & Always and Power Rangers Cosmic Fury.

Billy Cranston is a fictional character in the Power Rangers universe. He is the Blue Ranger (triceratops) in the series Mighty Morphin Power Rangers, and was portrayed by actor David Yost. Billy is the only original Power Ranger to remain for the entire MMPR series, and is the second longest-serving Ranger overall behind Tommy Oliver. Until his departure, he is considered the brains of the Power Rangers team, creating many gadgets with which to solve problems that not even Zordon foresees, and he even invents the first team's wrist-worn communication devices. A reimagined version of Billy appears in 2017 reboot film, played by actor RJ Cyler.