This book is written for high school and college students who are beginning their exploration of Christian liturgical history. Whether you are encountering these traditions for the first time in a theology class, a world religions course, or out of personal curiosity, this guide aims to make the rich and complex world of Christian worship accessible, engaging, and academically rigorous. The two liturgies examined here—the Divine Liturgy of St. Gregory the Great, which forms the foundation of the Western Roman Rite, and the Divine Liturgy of St. John Chrysostom, the primary Eucharistic rite of the Eastern Byzantine tradition—represent more than two thousand years of accumulated prayer, theology, art, and devotion. Together they account for the worship practice of well over a billion Christians around the world.