Charting a course between modalism (the belief that there is just one divine person) and tritheism (the belief that there are three divine substances or Gods) has long been the major problem for Trinitarian theology. In what follows, I shall discuss part of the contribution made by Duns Scotus to this problem. I will argue that, with a few small modifications, Scotus presents a coherent account of the doctrine of three persons in one substance, and thus that this doctrine can be coherently defended against both modalism and tritheism. I do not intend to give a complete presentation of Scotus's Trinitarian thought.