This thesis examines the influence of the teachings of the Theosophical Society, founded in 1875 by Madame Blavatsky, on the tradition of speculative and esoteric theories of music from its founding until the present day. Three fundamental propositions are identified: that music is of divine origin; that music can act as a moral determinate; and that music has healing and therapeutic powers. These speculative ideas have informed the Western occultic tradition throughout the centuries. The core teachings of the Theosophical Society are examined, and it is argued that while Blavatsky wrote only a minimal amount about music and metaphysics relating to music, her overall cosmological and occultic ideas had a profound influence on those musical thinkers and composers whose spiritual ideas were of an occultic leaning. The writings of various authors are examined, showing how these Theosophical notions have informed their views concerning speculative ideas of music. A number of contradictions within their writings are examined, as is the veracity of some of their claims. The primary focus is on the writings of Rudolf Steiner, Corinne Heline and Cyril Scott, as well as the effect of Theosophical teaching on the life and work of composers Alexander Scriabin and Gustav Holst. The final part of the thesis examines the ways in which Theosophical notions have been used and described from the 1970s until the present day under the rubric of the New Age Movement. It is contended that although these individuals interpreted Theosophical notions in various ways, they can all be categorised as having in common what Antoine Faivre, in his definition of esoteric, has called “a form of thought”. Faivre describes this as having six characteristics, four of which are essential: Correspondences, living nature, mediation, transmutation, praxis of concordance and transmission.