Ferruccio Busoni's conception of the musical work derives from his multiple roles as performer, aesthetician, editor, composer, arranger, and intellectual. Drawing on unpublished scores, manuscripts, sketches and documents from the Staatsbibliothek in Berlin, concert programs from a private collection in Berkeley, acoustic recordings, information about Busoni's intellectual interests gleaned from an auction catalogue featuring the contents of his extensive library, and the published aesthetic writings, letters, and compositions, the present study offers the first comprehensive account of Busoni's work concept. By establishing connections between his ideas and his musical practice, it explores and clarifies the reasoning behind his idiosyncratic compositional style, a style characterized by a blurring of boundaries between original and borrowed material. Polystylistic mixtures of the old and new and a distinctive performance style, in which Busoni creatively altered and embellished existing texts, exemplify his practice in an age in thrall to Werktreue, when originality of idea was prized above all else.