The best way to work out whether or not to believe in God is to compare the best theory that says that God exists with the best theory that says that God does not exist, taking into account all of the relevant data. This book compares Theism - the best theory that says that God exists - with Naturalism - the best theory that says that God does not exist - on a very wide range of data. The conclusion of the comparison is that Naturalism is a better theory than Theism: for Naturalism is simpler than Theism, and all of the considered data is explained at least as well by Naturalism as it is by Theism. The argument for Naturalism is novel both in outline, and in the details of the case that there is no data that Theism explains better than Naturalism does.