The Assumption of Mary refers to the ancient Christian belief that the Mother of God was taken up into Heaven, body and soul, at the end of her earthly life. For centuries, Catholics and other Christians celebrated the Assumption as a liturgical feast and meditated on the miraculous event in the Rosary. Nevertheless, its relationship to Revelation remained undefined. This changed in 1950 when Pope Pius XII declared the Assumption of Mary a dogma of the Catholic Church. In this rare exercise of papal authority, Pius XII infallibly and irrevocably taught that the Assumption was a truth revealed by God. This book explores how the definition of this Marian dogma came to fruition. After a brief history of the prior three Marian dogmas - Mother of God, Ever-Virgin, and the Immaculate Conception - this book narrates the major moments in the effort to obtain a dogmatic definition of the Assumption. The beginning of this ""Assumptionist movement"" can be dated to 1863 when Queen Isabel II of Spain petitioned the pope to declare the Assumption a dogma. Subsequently, petitionary efforts and scholarly inquiry increased and spread throughout the world. In addition to the narrative of this movement, this book gives special consideration to three vital aspects: debate over the Assumption's definability as a dogma, if and how Scripture reveals the Assumption, and the contribution of the laity on a matter of doctrine. Collectively, the Assumptionist movement emerges as a critical event in the nineteenth- and twentieth-century Catholic Church. Culminating in a dogmatic definition shortly before the opening of Vatican II, it also serves as a key point of inquiry for continuity and development of doctrine. A final chapter argues that the operative beliefs pertaining to Revelation, Scripture, and the laity during the Assumptionist movement stand in continuity with the teachings of Vatican II.