The Posterior Analytics is the summit of Aristotle's achievement in logic. It investigates the logical requirements for the most perfect of arguments, the demonstration, which proves a necessary conclusion from necessary premises. In his commentary on this treatise, Thomas Aquinas gives us perceptive interpretations of Aristotle's very concise and difficult text, together with illuminating explanations of the structure of the work as a whole and of the order of its parts. This new translation, based on the Leonine Commission's 1989 edition, seeks to render Aquinas's text faithfully in contemporary English. It includes a careful translation of the Latin text of Aristotle on which the commentary was based, with footnotes on passages where it differs from the Greek. To make the work as useful as possible for contemporary readers, the translator has provided an introduction and a supplementary commentary of his own. The introduction discusses three topics of fundamental importance for the study of the Posterior Analytics today: the relationship of Aristotle's logic to symbolic logic, the scope and subject matter of logic, and the status of the syllogism as an argument form. The supplementary commentary invites the reader to further reflection on the Posterior Analytics in the light of Aquinas's interpretation. Aquinas's commentary is divided into readings or lessons (lectiones), forty-four on Book I of the Posterior Analytics and twenty on Book II. The translator's supplementary commentary follows the same arrangement. The work includes footnotes, a brief bibliography of works cited, an index, and a preface by Ralph McInerny.