This dissertation argues that the eucharistic theology found in Thomas Aquinas's Summa Theologiae is not a Christocentric, static, hierarchical economy of grace production. Rather, it is a deeply Trinitarian, dynamic, communal drama of graced participation. Based on Aquinas's insistence that grace is a participation in the Divine Nature that is signified by the sacraments, I turn to the Secunda Pars in order to explicate the relationship between grace and human action that is presupposed in the sacramentology of the Tertia Pars. Insofar as the res tantum of the Eucharist is the unity of the mystical body of Christ, special attention is given to the relationship between grace, theological virtue, and moral virtue. Through close examination of the process through which charity is said to increase in the subject, the unity of the mystical body is seen, not as a mystical state, but as a graced action that is simultaneously God's action (insofar as grace formally moves us through charity) and the Church's action (insofar as the moral virtues dispose us to receive the presence of God as the extrinsic principle of our actions). The unity of the mystical body of Christ is, then, rightly called the grace of the Eucharist because the spiritual life affected by the Eucharist is the active presence of charity in the Church. The result of the Eucharist is the Church's participation in the Divine Nature. This project aims at providing a grammar that allows for fruitful dialogue in modern sacramental theology. Within Catholic Eucharistic theology, the scholastic language of metaphysics is regularly given place of privilege to such an extent as to view other grammars of the Eucharist with suspicion. This dissertation provides a Thomistic grammar of the Eucharist that largely avoids the traditional scholastic grammars. It is the hope that such retrieval is a catalyst for constructive dialogue between modern grammars (of all denominations) and traditional scholastic grammars.