Ptolemy's comprehensive treatises on astronomy and geography were influential for nearly two millennia. Equally influential was his treatise on harmonics, the ancient science which combined and brought to completion the study of philosophy and science. This volume offers a comprehensive English translation and commentary of Ptolemy's "Harmonics". The treatise begins with Ptolemy's study of pitches and intervals, for which he extracts both an idealized musical scale and a new acoustical tool. After discussing modulation, he expands his horizons by applying musical intervals to the human soul and celestial bodies, ultimately describing a cosmic harmony. The English translation seeks to reproduce Ptolemy's style faithfully and includes all the charts surviving in the manuscript tradition. The commentary offers a full exegesis of the text, loci paralleli, and citations of modern scholarly sources.