In this dissertation, I examine the role and evolution of diegetic music as represented in the Bühnenmusik of four pivotal operas: Richard Wagner's Tristan und Isolde (1865); Alban Berg's Wozzeck (1924) and Lulu (1935); and Bernd Alois Zimmermann's Die Soldaten (1965). These operas were chosen not simply for their shared Teutonic origins, but because they are important parts of an ideological and aesthetic lineage that clearly shows how the relationship between music and drama was transformed through the use of Bühnenmusik. In addition to exploring the musical materials and meanings of diegetic music within these operas, I address the challenges and peculiarities of conducting Bühnenmusik, and the role that evolving technology has played in overcoming issues of coordination between the stage and the orchestra pit.