Volume I. Part I describes the known versions of the Erec/Gereint story, giving whatever information is available about the circumstances of their composition. Particular attention is paid to the manuscript tradition of Erec et Enide by Chrétien de Troyes, to the place occupied in that tradition by the exemplar which was available to Hartmann von Aue, and to the two manuscripts of the French prose adaptation (showing the significance of the text contained in the unpublished Paris MS.). Part II is concerned with the highly problematical relationship between Erec et Enide and the Welsh story of Gereint fab Erbin. It is argued that the author of Gereint must have used a written source that was in a language other than Welsh. However, an important feature of Gereint is the technique of using formulas which, being Welsh, cannot have been taken from the (foreign) narrative source. There is evidence also of borrowing from a passage in the Historia Regum Britanniae, combined with material from Welsh tradition. Since the Welsh author used a technique of composition that will account for the differences between Gereint and Erec there is no advantage in supposing a lost common source. The disadvantages of such a supposition are that Chrétien's source may not have been a written text, and that it requires a belief in a whole series of coincidences to account for the total disappearance of the manuscripts. A final argument is available from the fact that Gereint incorporates information contained in a couplet which seems to be a later interpolation into the Erec text. Volume II contains the material (mainly text) which is to be read in parallel with the main discussion. The major item is an edition of Gereint fab Erbin (with English translation) marked in such a way as to show the different elements of its composition, and with corresponding passages from Erec et Enide set in parallel.