This book undertakes a literary and theological analysis of Gregory of Nyssa’s theology of union with God, culminating in a fresh reading of his final written work, In Canticum Canticorum. Part I provides the necessary background for the study of In Canticum Canticorum by analysing several of Gregory’s earlier works (c. 370 – 385), tracing the main contours of his account of the objective reality of human transformation and union with God. Part II uses the findings of Part I to guide and structure the analysis of In Canticum Canticorum (c. 391). Gregory’s allegorical interpretation of the Song of Songs is found to represent a corresponding account of human transformation and union with God from the perspective of subjective, affective experience of this reality. Rather than marking a new development in his theology, the subjective experience gained by Gregory’s reading of the Song of Songs recapitulates the key elements of Gregory’s objective account and therefore renders coherent his earlier soteriological doctrine.