Leo Ornstein (ca. December 2, 1893 – February 24, 2002), born in Ukraine (then a part of pre-Soviet Imperial Russia), was one of the leading American experimental composers and pianists of the early twentieth century. His performances of works by avant-garde composers and his own innovative and even shocking pieces made him a cause célèbre on both sides of the Atlantic. Ornstein was the first important composer to make extensive use of the tone cluster.