This dissertation argues that when properly translated, the twelfth-century concept of Eternity cannot merely be defined by what it lacks, that is, Time, but by what it is, in so far as it is possible to articulate it. Twelfth-century thinkers, here considered from 1075 to 1170, used the concepts of Eternity and Time to articulate the nature of God and to pursue the wholeness of being, life, and knowledge amidst the seeming fragmentation of the temporal world. Their thought on this subject was informed by both the writings of church fathers, especially Augustine, and Latin translations of Platonic philosophy. The first section examines two twelfth-century theological masterpieces that clearly cite Augustine's De Trinitate as their primary sources. Chapter one begins with Anselm's Monologion (1075-1076), one of the most well known medieval articulations of atemporal Eternity. This work is significant because it articulates the concept of Eternity as Divine Maximum and then applies this transcendent concept to divine immanence. Chapter two argues that Richard of St. Victor's De Trinitate (1170) develops Anselm's theories of Eternity as Divine Maximum in a Trinitarian direction by articulating more fully what it means to be a Trinity of co-eternal persons. The second section looks at twelfth-century works that are inspired by Platonic sources, primarily Calcidius' translation and commentary on Plato's Timaeus, Macrobius, and Martianus Capella. Chapter three introduces the key hermeneutic problems related to divine Eternity present in these twelfth-century sources for Platonism. Most notable among these problems is that Platonic sources did not always consider the Maximum God to be eternal, even though the scriptures and Augustine presented the entire Trinity as eternal and maximum. How then do twelfth-century thinkers reconcile the Platonism they see in these texts with orthodox Christian beliefs? Chapter five looks at how and why Bernard of Chartres (d.1124), William of Conches (fl.1120–1145), and Peter Abelard (1079–1142) address the problems presented in chapter three. Chapter six, the final chapter, presents the conception of Eternity in the metaphysics of Thierry of Chartres (d.1150) as a philosophical tour de force, which not only reconciles Platonism with Christian orthodoxy but also relates Platonic and Aristotelian ideas through Stoic Logic. To do this, Thierry applies Pythagorean mathematics to the Trinity and employs the concept of universitas as a category that spans Eternity, Perpetuity, and Time. It is then suggested that this text may have influence Richard of St. Victor's De Trinitate.