This chapter examines the relationship between medieval theology and Reformed theology. The Reformation broke with medieval thought upon key issues, but in many areas, medieval influence remained. Many of the Reformers, including Luther, Zwingli, Bucer, and Vermigli, were trained in various forms of medieval thought, such as Thomism, Scotism, and Nominalism. Calvin was unique in that he was mostly self-trained in theology, although the nature of his training remains disputed. As Reformed theology became institutionalized, medieval theology became a valuable tool in the defence of Reformed thought, as exemplified in the writings of John Owen and Francis Turretin. The chapter also examines Reformed theology in the areas of the doctrine of God, of providence, of predestination, of sanctification, and of the person of Christ, noting the influence of medieval thought upon the formulation of these doctrines.