Generally acknowledged as the most important German musicologist of his age, Hugo Riemann (1849-1919) shaped the ideas of generations of music scholars, not least because his work coincided with the institutionalisation of academic musicology around the turn of the last century. This influence, however, belies the contentious idea at the heart of his musical thought, an idea he defended for most of his career - harmonic dualism. By situating Riemann's musical thought within turn-of-the-century discourses about the natural sciences, German nationhood and modern technology, this book reconstructs the cultural context in which Riemann's ideas not only 'made sense' but advanced an understanding of the tonal tradition as both natural and German. Riemann's musical thought - from his considerations of acoustical properties to his aesthetic and music-historical views - thus regains the coherence and cultural urgency that it once possessed.