This work examines the probable extent and nature of interpolation in Thucydides. It aims to provide a typology which may be of some use in the study of other manuscript traditions and which will clarify hard passages, many of which are discussed in depth, so that the book’s Index Locorum can be a tool for students of this author. Separate chapters examine evidence given by manuscript disagreement, by a long inscription, by papyri, by scholiasts, by Valla’s translation, and more. A chapter analyzes the types of mechanical interpolation ; another looks at the hypothesis of Hellenistic editing . Constant close attention is paid to the stemma codicum (discussed also in an appendix) and to the smallest idiosyncrasies of Thucydides’s style.