This study focusses on the writing of history in medieval Wales. Its starting-point is a series of historical texts in Middle Welsh which, from the second quarter of the fourteenth century, begin to appear together in manuscripts to form a continuous history, termed the Welsh Historical Continuum. The central component of this sequence is a translation of Geoffrey of Monmouth’s influential history of the Britons. The main questions of the first part of the thesis are when and why these historical texts were first combined, and to what degree this Welsh historiographical phenomenon reflects broader European trends. Codicology, textual typology, a geographically centred case-study and comparison with similar texts in Latin, Anglo-Norman French, Middle English and Icelandic are the main areas of research. The second part of the thesis moves on to consider the chronicle writing which formed the basis for the third part of the Historical Continuum, and then brings the study together with a discussion of the role of the Cistercians in the writing of history in medieval Wales. The fourth chapter’s re-assessment of Brut y Tywysogion offers a comprehensive re-evaluation of one of the most important narrative sources for medieval Wales. The fifth chapter discusses a neglected but significant Welsh chronicle, O Oes Gwrtheyrn, a new edition of which is appended to the thesis. The discussion of the Cistercian order in the sixth chapter serves in some ways as a synopsis and a conclusion, since it fits the diverse matters discussed in previous chapters into a general discussion of the important role these monastic institutions played in the formation and dissemination of what became the standard narrative of Welsh history for several centuries. Overall, the thesis is a wide-ranging and comprehensive investigation of the most influential and enduring historical narrative to emerge from medieval Wales.