William E. Caplin (born 1948) is an American music theorist who lives and works in Montreal, Canada, where he is a James McGill Professor at the Schulich School of Music of McGill University. Caplin served as president of the Society for Music Theory from 2005 to 2007 and was its vice-president from 2001 to 2003.
Although much of Caplin's earlier work concentrated on the history of music theory (his dissertation was on theories of meter), he is best known for his so-called "theory of formal functions." His book Classical Form: A Theory of Formal Functions for the Music of Haydn, Mozart, and Beethoven has been widely influential and has been a major factor in the revival of interest in musical form in North-American music theory.
- William Caplin faculty page at McGill University