The poems of Catullus have notoriously been subjected to numerous accidental corruptions. This work represents a radical reappraisal of his text. It recommends some six hundred changes to the Oxford Text of R.A.B. Mynors; many of these proposals are easily accessible elsewhere, but many are either original or else more or less forgotten. It is suggested here that Catullus' text was also subjected to significant deliberate change, much of it probably dating back to classical antiquity. These changes consist in part of around seventy interpolated lines, often designed to explain or paraphrase what Catullus had written, and in part of modernizations designed to adapt a Republican poet, the near contemporary of Cicero and Lucretius, to the poetical norms of the early Empire. Students of Catullus will certainly wish to take account of the arguments here advanced, even where they find themselves in disagreement with the conclusions.