Nikolai Andreyevich Rimsky-Korsakov (1844-1908) was celebrated during his lifetime as a composer and instructor, and his musical works and publications on instrumentation remain prominent today. However, his innovations as a music theorist have gone largely unrecognized. Rimsky-Korsakov’s Harmonic Theory is the first comprehensive study of the composer’s unique concept of harmony. Larisa P. Jackson illuminates Rimsky-Korsakov’s harmonic theory and reveals the intellectual, social, and cultural facets of its historical contexts in both Western and Russian music. In this unprecedented contribution to musicology and music theory, Jackson examines and clarifies Rimsky-Korsakov’s thinking on modulation (key changes), which composers began using much more frequently during the nineteenth century. Based on his discovery of a previously unknown scale, Rimsky-Korsakov saw modulation as shaped by a web of deep relationships among major and minor keys. Jackson charts this tonal space, mapping its implications as well as its often-surprising relationships with the theories of Rimsky-Korsakov’s predecessors and contemporaries, including the famous German music theorists Hauptmann and Riemann.