For modern audiences, Merlin has had an enduring appeal as the wizard at the court of King Arthur. In the Middle Ages, his fame was if anything even greater, but his supporting role alongside Arthur was only one part of it, and neither the first nor necessarily the most important part at that. Merlin was a figure of fascination in his own right, someone to whom medieval writers returned for a variety of reasons, over centuries of retelling and reinterpretation. This process might be likened to a great river, rising from several springs, and ultimately flowing out in many different streams, as the stories of Merlin and of Arthur spread across Europe and down centuries. It is obviously beyond the scope of the present essay to examine in detail all of the medieval texts in which Merlin appears, but the main course of this ‘river’ is easy enough to trace. From scattered sources in Welsh myth and the ninth-century Historia Brittonum, it is first recognisable in Geoffrey of Monmouth's Historia Regum Britanniae (hereafter Historia) and passes from there into the literary chronicles that were its direct descendants, such as Wace's Roman de Brut and Laʒamon's Brut.