This work examines the role of the doctrine of 'divine ideas' in the theology of Thomas Aquinas, a question which remains controversial. Aquinas received this doctrine in two distinct forms, from Augustine and Dionysius. The historical origins and development of this twofold tradition are traced from Plato and Aristotle, through Hellenistic philosophy, to the patristic and medieval periods. In Aquinas' account of God's knowledge, of the Word of God, of Creation and of Providence the doctrine of divine ideas plays a key role. Various strands of neoplatonist thought are clearly important for him but it is Aristotle who is of greatest significance for Aquinas' sustained and original re-thinking of the doctrine. A study of this question provides a fresh perspective on the nature of Aquinas' unique synthesis.