William van Roo's declaration thirty years ago that "there is no full synthetic account of [Augustine's] thought" on sacramentum still rings true to this day. Already in 1953, Charles Couturier had conducted the most extensive study. But Couturier identified his own work as more of a sketch than a synthesis. His was more of a word study than an account of the concept of sacramentum as it fits into Augustine's thought. This lacuna in Augustinian studies is most unfortunate. But it is also understandable. There are at least two major reasons it is difficult to provide a composite account. First, Augustine's sacramentum comes in the midst of a vast tradition of "sacramental theology." What precedes Augustine is already dynamic and differentiated but less well known. What follows bears Augustine's own imprint; it is a tradition modern readers are powerless to bracket entirely. But upon closer examination, Augustine's use of sacramentum is more integral to, and indicative of, the development of Augustine's theology than his forebears and it is more capacious and fluid than the sacraments known even to medieval doctors of the Church. Second, the concept of sacramentum and its broader significance stands on a fault line in Augustinian studies. Scholars agree that Augustine's sacramentum suggests some kind of relationship between visible signs and the interior realities they signify. But this suggestion elicits two rather different responses. Readers convinced Augustine never escapes the dualism of Platonism or Manichaeism doubt his ability to affirm this union. In contrast, many who take Augustine to move beyond his dualistic influences see in the centrality of signification to his theology a fundamental "sacramentality," "sacramental principle," or "sacramental worldview." The first response ascribes to Augustine a contradiction of which I will argue he is not guilty; the second I follow in spirit, but seek to qualify with a more concrete analysis of sacramentum that overcomes the tendency towards abstraction among scholars of this persuasion.This dissertation provides a synthetic account of sacramentum that surpasses the constraints of a mere word study to ascertain its meaning and significance to the developments and eventual structure of Augustine's theology. My principal aim is to show how at the heart of Augustine's theology, underlying his definition and defense of Christianity as the true religion, lies an account of God's economy of salvation for which "sacraments" are constitutive. I analyze the backgrounds and anticipations, the establishment, development, and stabilization of what I call the "sacramental" economy of salvation in Augustine's theology. By "sacramental economy" I mean God's purposeful arrangement of visible sacraments in creation, Scripture, and the Church for the sake of human salvation. Although the sacraments of baptism and the Lord's Supper are crucial, the sacramental economy includes the full range of what Augustine means by the term sacramentum (and its synonyms). For Augustine, the "economy" is the order, arrangement, and historical enactment of God's eternal plan for human salvation. As Augustine's own theology developed, there was a shift in the power he found in the concept of sacramentum for describing God's temporal disclosure and fulfillment of his eternal plan of salvation, especially as encapsulated in Christ's Person, work, and union with the Church.Implicit in my argument is the contention that Augustine's theological development is largely the story of his reckoning with the mystery or sacrament of Christ. The creative Word of God becoming flesh (Jn. 1:14) and humbling himself to the point of death on the cross (Phil. 2:8) is, for Augustine, the Sacrament par excellence. By the time he becomes the Bishop of Hippo (395/6), Augustine interprets the Christ event to be the paradigm for how God has arranged the sacraments of creation, Scripture, and the Church into an economy constructed for the sake of human salvation. In the years leading up to this point, Augustine gradually internalizes this sacramental economy as a deep structure of his Catholic faith, a proper sense of created reality as given and made known by God. Augustine's conception of the sacramental economy reaches stability around the time of his appointment as Bishop of Hippo and remains steady at least until his encounter with the Pelagians (411/2), the terminus ad quem of this study. At the beginning of this period, Augustine formulates a more unified account of divine grace and human willing on the one hand and of the relationship between divinely given signs (i.e., sacraments) and what they signify on the other. His elaboration of Paul's magnum sacramentum in terms of the totus Christus clarifies the normative significance of Christ's mediation in the sacrament of cross, but it also explains the pattern of sacraments as unifying what can appear to be related by suggestion or signification alone. In the end, I argue, Augustine's mature theology of the sacramental economy evinces an order of sacraments that descends from (1) the crucified Christ through (2) the great sacrament of Christ's union with the Church to (3) the sacraments of worship that incorporate believers into Christ's death and this union, and ends with (4) the sacraments of the word in Scripture and in creation as understood in the light of Scripture.