A historical and systematic introduction to what the medieval philosopher and theologian Thomas Aquinas wrote about the Trinity. By focusing on the thought of one of the greatest defenders of the doctrine of the Trinity, Gilles Emery OP elucidates the classical Christian understanding of God. Not only does it constitute a remarkable introduction to the central intuitions of the trinitarian synthesis of the Summa theologiae, but also, in dialogue with contemporary trinitarian issues, leads the reader to a refined intelligence of this mystery-source, at the school of St Thomas Aquinas.