This thesis presents a new edition of the major recensions of the Historia Brittonum. It is the first to depart from the pattern of conflated texts which has been followed by editors since 1691. Each may now be read as a text in its own right. I have argued that the 'Harleian' recension is the primary version of the Historia Brittonum and belongs to the year 829/30, and have shown that the attribution of the work to one 'Nennius' is late and unacceptable. The complicated textual tradition has been examined, from this early-ninth-century origin, throughout its mediaeval history; the fullest development is seen in the 'Sawley' recension of the beginning of the thirteenth century. I have also considered the early modern tradition of the work, represented by a large group of paper manuscripts prepared by or for the antiquaries of the sixteenth and seventeenth centuries as no printed text was available until 1691. In addition to detailed studies of manuscripts and textual tradition, I have prepared a literal modern English translation of the primary recension and have made a detailed preliminary study of its latinity. My remarks on the later recensions concentrate on establishing the filiation of the manuscripts and on placing each new version within the context of the textual tradition as a whole. This has seemed to be the primary requirement in any new investigation of the Historia. Work can now go forward, from a secure textual base, on the implications of this important series of texts for historical and literary studies.