Leonard David Stein (December 1, 1916 – June 2004) was a musicologist, pianist, conductor, university teacher, and influential in promoting contemporary music on the American West Coast. He was for years Arnold Schoenberg's assistant, music director of the Schoenberg Institute at USC, and among the foremost authorities on Schoenberg's music. He was also an influential teacher in the lives of many younger composers, such as the influential minimalist La Monte Young.