Two themes have dominated philosophical discussions concerning the rationality of believing a report of a miracle. The first relies on the idea that miracles are by definition massively improbable. The second theme involves the thought that testimony is, in general, not a very reliable source of information. The result of combining these two themes is that it is very difficult – some suggest impossible – to rationally believe that a miracle has occurred on the basis of testimony: on its own, testimony is too weak to outweigh the improbability of a miracle. Both themes are addressed in Hume’s famous essay on miracles. This chapter examines each theme and critically discusses interpretations of and replies to Hume’s argument.