Theodor Mommsen's edition of Gildas Sapiens' De excidio Britanniae (1898) was accepted almost universally as the standard throughout the twentieth century, and his assessment of the DEB manuscript tradition was very rarely challenged. The main aim of Larpi's study is to reconstruct and assess Mommsen's recension of the DEB tradition, not only with reference to those witnesses known to and used by him, but also in the light of the discovery of a new manuscript of the work (Reims, Bibliothèque Carnegie, Ms. 414), and two new indirect sources (the 'Leiden' and 'Corpus' glossaries). After an introduction to Gildas, the manuscript tradition, the indirect sources and the editions of DEB are analysed. It is argued that a key point in Mommsen's assessment of the value of the individual manuscripts is the relationship between Bede and the tradition, and this view is tested by a systematic analysis of Bede's use of Gildas, in order to establish whether, and to what extent, the writings of this author contaminated some of the witnesses to the text of Gildas. On the basis of the results of this analysis a new stemma codicum is drawn in an attempt to propose a new recension of the surviving tradition. In the final section of the study, the case for a new edition is made, on the strength of the collation of Reims Ms. 414 with the rest of the tradition.