Harley 3859 contains not only the earliest but also the best-known medieval collection of Welsh genealogies. Its fame stems from the rare light that it casts upon the early medieval period in Wales. It is written in a combination of Old Welsh and Latin and concerns itself with the Brittonic dynasties that held power in Wales and northern Britain up to the middle of the tenth century. The text is one of the few direct historical sources for Welsh dynastic history in the early Middle Ages and for Welsh views of the northern Brittonic past during the crucial years of the sixth century. In many ways, the ‘Harleian genealogies’ provide the touchstone for the study of all later Welsh genealogical texts, because they show the form that such texts would have taken during the earliest discernible period of their production, during the ninth and tenth centuries.

The importance of the Harleian genealogies as a historical source has motivated some close studies of the text. The groundwork was laid in 1888 by Egerton Phillimore, who appended his pioneering discussion with an outstanding diplomatic edition. Another edition, with accompanying notes, was provided by Peter Bartrum in 1966. Major studies were undertaken by E. W. B. Nicholson in 1908 and David Thornton in his 1991 PhD thesis. Shorter considerations have been included in general histories of the period, such as J. E. Lloyd's History of Wales, Wendy Davies's Wales in the Early Middle Ages and T. M. Charles-Edwards's Wales and the Britons. These contributions, alongside many other more limited studies noticed elsewhere in this volume, have significantly enhanced our understanding of the extant text. The more recent work has generally focussed on the political implications of the genealogical collection in its mid-tenth-century context, when it was redacted during the reign of Owain ap Hywel Dda, king of Deheubarth (c. 950–88). This mid-tenth-century level of redaction is called here the ‘St Davids recension’, on account of the probable location where the redactor worked. The ‘Harleian genealogies’ in Harley 3859 are effectively the only full and direct witness to this St Davids recension, even though there is a considerable body of evidence to suggest that other copies of the St Davids recension circulated in Wales and beyond during the following centuries.