In 1944, Thomas Parry published, in Welsh, his ‘History of Welsh Literature to 1900’. It was a work whose scope was unparalleled by previous literary histories, covering thirteen centuries of literature in Welsh. In one volume, Parry created for Welsh speakers a comprehensive literary canon that not only defined the aspirations and identity of a cultural group but also called a nation into being. This article explores some of the reasons why the medieval is privileged in the Welsh literary canon, and how cultural and political authority, both Welsh and English, has worked to ensure the stability of the medieval canon for more than two centuries. The efforts of Welsh literary historians to naturalize the medieval Welsh canon were supported by English critical opinion in the nineteenth and early twentieth centuries, and the article looks in particular at the work of Matthew Arnold and Hector and Nora Chadwick, arguing that they played a significant role in validating the canon of medieval Welsh literature.