This chapter focusses on Aquinas’s philosophical arguments for the existence of God. It begins by noting where he offers such arguments. Then it notes his general strategy in presenting them. Unlike many philosophers, Aquinas argues that the claim that we can only know that God exists by means of demonstrative arguments from certain facts about the universe to the conclusion that God must exist as cause of these facts. This chapter explains what that view amounts to for Aquinas and compares and contrasts it with other philosophical approaches to the question ‘How can we know that God exists’. The chapter then turns in detail to Aquinas’s arguments for God’s existence in his De Ente et Essentia (4) and his Summa Theologiae, 1a, 2, 3. The chapter explains how these arguments run and comments on what can be said for and against them.