This dissertation studies Thomas Aquinas's doctrine of theological hope, on its own terms and in its full context, in order to elucidate hope's essence and significance in the Christian life. The study is undertaken through a close reading of the Summa Theologiae according to which Aquinas's doctrine of theological hope is related to his doctrines of the divine ordinatio, human beatitude, providence, predestination, the Trinitarian missions, Christ, grace, sin, the theological virtues, the natural desire for happiness, human free choice, and the passions. It also attends to the way in which Thomas develops his teaching on hope over the course of his career. This research addresses a lack of sustained attention to Aquinas's doctrine of hope within contemporary scholarship, especially regarding its evolution and significance within his larger account of God and God's activity ad extra. In Thomas's mature teaching, hope is not an ambiguous middle term between faith and charity. The theological virtue of hope is a person's holy self love and desire for eternal beatitude, as well as her trust in God's merciful and omnipotent love, and her voluntary reception of his grace. With charity, hope is the Holy Spirit's assimilation of the human will to his eternal procession from the Father and the Son by way of love, and a fundamental aspect of the gradual restoration and perfection of the imago Dei. Thomas's doctrine illuminates why hope is most perfectly expressed in the prayer of the "Our Father," and manifested in the life of the Gospel beatitudes. Charity and meritorious acts depend on hope, and so it is the origin of every step on the human being's journey to God. In Thomas's account, hope for oneself is always hope for one's brothers and sisters in the body of Christ, and hope for others plays an indispensable role in their salvation. Furthermore, Thomas's understanding of hope's significance in preserving the order of creation luminously reveals God's wisdom and love in the dispensation of salvation.