This dissertation explores Thomas Aquinas' complex teaching on Jesus Christ as priest through close readings of the primary texts, attentiveness to the numerous ways he structures this teaching, and persistent investigation into its development over the course of his career.  Chapter 1 provides an overview of Christ as both priest and mediator in Thomas' Summa theologiae and dispels confusion between these roles through recourse to Thomas' scriptural commentaries. Chapter 2 proceeds from the perplexing separation of those questions on Christ as priest and Christ as mediator to argue that Thomas structured the entire first section of questions on Christ in the Summa theologiae around the concept of mediation.  Chapter 3 then charts the development of Thomas' teaching on sacrifice within the Summa theologiae. Undoubtedly influenced by Augustine's City of God, Thomasmoves from distinguishing martyrdom and other virtuous acts from proper sacrifices to accepting them and, especially, Christ's own martyrdom as sacrifices in the fullest sense. Chapter 4 returns to issues of structure and uses the conclusions of Chapter 2 to discern a priestly inspiration in the second section of questions on Christ in the Summa theologiae. Chapter 5 then considers participation in Christ's priesthood and sacrifice through the sacraments. It contends that the sacramental character of confirmation bestows on the faithful an office of self-offering in the celebration of the Eucharist, in which emphasis is placed on the spiritual priesthood and sacrifices of the faithful, the ordained priest standing "in the person of the Church," and Christ's perpetual self-offering leading the faithful to their own perfect access to God. Finally, Chapter 6 argues that a profound development on this topic is discernible from some of Thomas' earliest works (Commentary on the Sentences; De veritate), through the middle of his career (Summa contra Gentiles; Compendium theologiae; Commentary on Hebrews), to his more mature work (De perfectione; Summa theologiae). Through a new exegesis of Heb. 5:1, Thomas secures his mature teaching on Christ as the priest and mediator Who offers Himself as sacrifice to His Father and as gift to human beings.