This special issue of Harvard Theological Review is devoted to a critical discussion of fourth-century Christian trinitarian theology, a topic that is now in a significant new phase of scholarly debate amongst both historical and systematic theologians. The papers and conversation published here arose from a day-conference on 5 May 2006 at Harvard Divinity School, when a number of invited scholars and doctoral students from Yale, Chicago, Emory, Fordham, Weston Jesuit School of Theology, and St. Vladimir's Orthodox Theological Seminary, joined the students of the Harvard conference course, “Trinitarianism and Anti-trinitarianism: The Christian God in Dispute” (Spring 2006), for a day of shared papers and public debate. The immediate focus of the event was a roundtable on Lewis Ayres's important new book, Nicaea and Its Legacy (Oxford: Oxford University Press, 2004), and that discussion—in extended format—now makes up the first part of this issue. Other papers from students then followed, supplemented by comments from senior members from the floor. In the second part of this issue, two of those original papers, along with two other specially commissioned pieces—on Gregory of Nazianzus and Augustine, respectively—extend and refine the debate outlined in the first part. This brief introduction will explain the wider significance of this ongoing debate about patristic trinitarianism, both East and West, and outline what this issue of HTR contributes to it.