This present thesis examines the manuscript tradition of the family y of Procopius' Wars Books VNIII, in the light of a hitherto unknown extant manuscript (Ath), Athos, Lavra codex H-73 (728). It comprises an introduction and four sections (1-4). The introduction gives information on the author and his works concentrating on his major historical work, the Wars. This is followed by Section 1 containing an analytical codicological and palaeological description of codex Ath. As the codex does not contain a colophon, a possible thirteenth/fourteenth century dating is based on palaeographical and codicological evidence. Section 2 examines the position of the codex in the stemma codicum, proposed by the latest editor of the text, Jacob Haury (1905-12). A collation of the text with the principal manuscripts (K and L) of the two families, z and y, shows that Ath belongs to the y family. A further collation of Ath with all other extant manuscripts of this family, illustrates the importance of Ath in the tradition of the text. Section 3 gives a description and updated information of all manuscripts of family y, before the relation is examined and the stemma codicum is reconstructed on the basis of a series of propositions. The thesis concludes with Section 4, which places the production of Ath and other manuscripts, containing Procopius' works and other early Byzantine historiographical texts, in the general context of the intellectual milieu of the Palaeologan period.