As a distinct genre and subject, twentieth-century violin and viola duos have not been examined in depth or detail. Existing catalogues which list works for this ensemble are imcomplete and historical information concerning the genre as a whole is sparse. Only a few brief articles concerning twentieth-century violin and viola duos and one analysis of a work have been publised. The genre is even considered by many to be of non-standard instrumentation. Yet, the combination of violin and viola is unquestionably viable and has been chosen as a means of expression by at least 360 composers during this century alone. Over 400 modern works for the combination have been written and scores and/or parts for approximately 200 of these duos are readily available. In addition, recordings of 26 compositions have been released commercially. This dissertation addresses the under-researched topic of twentieth-century violin and viola duos. First it is show that the topic has not been adequately studied and possible reasons for this are given. Second, the musical function of duos is discussed and a historical groundwork is established which traces duo literature from the seventeenth century. An overview of the modern violin and viola duos follows. Next, critical reviews of 202 works and a discograpy of recorded duos, both commercial and non-commercial, precede a list of titles, composers, publishers and referential listings of over 400 violin and viola duos. Finally, analyses of seven of the more prominent twentieth-century violin and viola duos are presented.