Ontological arguments are one of the main classes of arguments for the existence of God, and have been influential from the Middle Ages right up until the present time. This accessible volume offers a comprehensive survey and assessment of them, starting with a sequence of chapters charting their history - from Anselm and Aquinas, via Descartes, Leibniz, Kant and Hegel, to Goedel, Plantinga, Lewis and Tichy. This is followed by chapters on the most important topics to have emerged in the discussion of ontological arguments: the relationship between conceivability and possibility, the charge that ontological arguments beg the question, and the nature of existence. The volume as a whole shows clearly how these arguments emerged and developed, how we should think about them, and why they remain important today.