Geoffrey of Monmouth in his History of the Kings of Britain is widely considered to have transgressed the historiographical canons of his time. The work provides a lengthy and detailed account of a prehistoric period for which no history in any currently accepted sense can be written. Among Geoffrey's greater departures from historical credibility is his championship of two mythical figures, Arthur and Merlin, both of whom are given a central place in his History. In this article, the author considers the evidence for the reception of Merlin's Prophecies and its implications for the reception of the history in which they were located. Besides reviewing the testimony of twelfth-century authors who used or criticised the Prophecies, she looks at commentaries on the Prophecies, both published and unpublished, written by contemporaries. She concludes that the Prophecies were attacked not because of any perceived historical inaccuracy but primarily because of political considerations. Indeed, the presence of Merlin's Prophecies at the heart of the History served to enhance its credibility and validity.