Few terms in music theory are more profound and more enigmatic than tonality. First coined in the early nineteenth century, the term denotes music that focuses melodically and/or harmonically on some stable pitch or tonic, especially functional triads. The term has since become an essential part of theoretical discourse. Of the many attempts to explain the nature of functional tonality, perhaps the most comprehensive was undertaken by Heinrich Schenker (1868-1935). In his monumental triptych Neue musikalische Theorien und Phantasien (1906-1935), he systematically investigated the ways in which lines and chords behave both locally within individual tonal phrases and globally across entire monotonal compositions. This book tries to show why Schenker was able to explain tonal relationships so successfully and why his explanations have advantages over those of his rivals. The volume is subdivided into an introduction and six chapters. The introduction explores some of the general methodological issues that arise when we try to build, test, and evaluate a plausible theory of tonality. It begins by outlining the main ingredients of such a theory, namely concepts, laws, and procedures, and describes some of the problems that they raise. The introduction goes on to discuss six criteria that theorists typically use to evaluate the success of their models. These criteria include accuracy, scope, fruitfulness, consistency, simplicity, and coherence. With this broad theoretical framework in place, the central portion of this book uses these criteria to illuminate the foundations of Schenkerian theory. The conclusion describes some of the ways in which Schenkerian theory might develop in the future. Matthew Brown is Associate Professor of Music Theory at the Eastman School of Music, University of Rochester.