This monograph is a critical and historical account of Aristotelian essentialism and modal logic. In Chapter One, ancient and contemporary interpretations and claims of inconsistency in Aristotle's modal syllogistic are examined. A more consistent model is developed through attention to Aristotle's comments on negation. In Chapter Two, proofs for each of the mixed apodictic syllogisms are analyzed and diagrammed. Chapter Three explores how Aristotle's modal metaphysics fits within the context of the Posterior Analytics. Chapter Four contrasts Aristotelian modal logic to contemporary modal metaphysics and argues for ways in which a return to Aristotle may spark intriguing thought in contemporary discussions of the philosophy of science and in debate over the metaphysics of identity.