Isaac Casaubon’s treatise De critica was apparently completed, but it was never published, and no manuscript of the work has come to light. Since it appears to have been a substantial work on textual criticism by one of the most eminent and capable scholars of the period, its loss is tantalising. This article uses new manuscript evidence to throw light on its content and purpose. Five pieces of manuscript evidence are presented here. Three of these are documents which Casaubon himself (...) composed while preparing the work: two leaves outlining the contents of the work, and two sets of notes intended for the first book. The final part of the article will look at the role of Johannes Woverius of Hamburg (1574–1612) in the later fortunes of this work and publish a suppressed passage from a letter of Casaubon showing that Woverius had seen a copy of the treatise. Many of the editions used by Casaubon when making his notes are identified below, as are a number of printed books annotated in Casaubon’s hand. This new evidence illustrates the evolution of Casaubon’s early scholarship in the 1580s and 1590s. It allows a first description of his Observationes, an unpublished work which lay beneath, and evolved into, De critica. An outline of De critica enables an assessment of the place of Casaubon’s lost work in the critical literature of the late Renaissance, and a new account of its relationship to an extant work, Woverius’s treatise De polymathia. (shrink)