Long recognized as one of the most important medieval treatises on music, the Musica of Hermannus Contractus is here presented in a newly revised translation, with commentary reflecting the best current scholarship. A polymath and monk, Hermannus Contractus (1013-54) contributed to the important advancements made in European arts and sciences in the first half of the eleventh century, writing on history, astronomy, and time-keeping devices, among other topics, and composing several chants. His music theory, founded on a systematic treatment of traditional concepts and terminology dating back to the ancient Greeks, is concerned largely with the organization of pitch in Gregorian chant. Hermann's approach stems from Germanic species-based thought, and is marked by a distinction between aspects of form and aspects of position, privileging the latter. He expresses this in terms imported from then-new developments in Italian music theory, thus acting as a nexus for the two traditions. Numerology and number symbolism play significant roles in Hermann's theories, and his critiques of other theorists offer insights into medieval intellectual life. Hermann also uses chant citations and exercises to help his readers apply theory to practice. John L. Snyder's revised edition of Ellinwood's long-standard 1952 text and translation offers a new introduction, including codicological descriptions of the sources; a critical edition of the Latin text with an annotated English translation on facing pages; appendices detailing the documents pertaining to Hermann's life, his citations of plainsong, and his original diastematic notation system; and greatly expanded indexes. Snyder's Musica will serve as the standard version of this major historical document for years to come. Leonard Ellinwood (1905-94) served in the Library of Congress cataloging divisions in music and in the humanities for thirty-five years. He published scholarly works and editions of both medieval music and church music. John L. Snyder is Professor of Music Theory and Musicology at the University of Houston's Moores School of Music.