Abstract: William O. Smith's contributions to the musical landscape of the twentieth century are immense. As a composer, he has been the recipient of several prestigious awards, including: a Guggenheim Fellowship, Prix de Paris, Prix de Rome, and Phelan Award. His development of a card catalogue of multiphonics for the clarinet in 1960 was revolutionary, and profoundly influenced many composers such as Gunther Schuller, John Eaton, and Eric Mandat; who have all dedicated works to Smith. As a jazz artist, his recordings with Dave Brubeck (and other prominent jazz artists) were also innovative, and showcase some of the first Third Stream compositions (along with remarkable jazz clarinet performances). This research project traces his life through each important phase of his career (in both composition and performance). Each section includes reviews, correspondence, and interviews that highlight his professional accomplishments during that period. The main areas of focus are: his early studies with Darius Milhaud and Roger Sessions (and poly-tonal approach to composition), membership in the Dave Brubeck Octet (and continuing relationship with Dave Brubeck), shift to serial techniques (during his time at the American Academy in Rome), invention and documentation of multiphonics for the clarinet, work with the American Jazz Ensemble, the double and demi-clarinet, poly-tempo concepts, aspects of theatre in composition, minimalism, electronics, and the combination of jazz and classical idioms. A complete list of his compositions and a discography are included in the appendices.