The doctrine of divine simplicity is largely ignored in modern continental theology and has been criticized by some analytic theists. However, it plays a central role in patristic and medieval trinitarian thought, and is a doctrinal affirmation of the Catholic Church. This article seeks to illustrate the significance of the teaching first by examining the contrasting modern trinitarian theologies of Karl Barth and Richard Swinburne, noting how each suffers from a deficit of reference to the doctrine of divine simplicity. The article then presents four aspects of Aquinas’ teaching on divine simplicity. From this a consideration of trinitarian persons ensues that illustrates why the distinction of persons in God can best be understood by making use of Aquinas’ theology of ‘subsistent relations’, while the unified nature of God can best be understood in terms of ‘personal modes of subsistence’. Based on this analysis, the contrasting insights of both Barth and Swinburne can be fully retained, without the contrasting inherent problems that the theology of each presents.