It has often been claimed by philosophical theologians that the concept of God functions as an ‘ultimate explanation’ of the nature of the universe. In recent years, various theologians have regarded this notion of ‘ultimate explanation’ as one which has a central place in religious belief; and they construe the concept of God, at least in part, and sometimes mainly, as a concept which provides such an explanation. On the other hand, from the time of David Hume there has been a long line of eminent philosophers who have found the whole idea of an ultimate explanation—something which completely explains absolutely everything—an incoherent one, analogous to the idea of the square of a circle. I think it worth trying to explore the possibilities and limits of the notion, and attempting to assess its importance for religious belief. First, I will briefly sketch the notion as it is found in recent work, and then offer an analysis of the notion of ‘explanation’ in general. Finally, I will try to ascertain the nature and limits of the notion in religion.