The Metaphysics of Gender is a book about gender essentialism: What it is and why it might be true. It opens with the question: What is gender essentialism? The first chapter distinguishes between essentialism about kinds of individuals (e.g. women and men as groups) and essentialism aboutindividuals (e.g. you and me). Successive chapters introduce the ingredients for a theory of gender essentialism about individuals, called uniessentialism. Gender uniessentialism claims that a social individual's gender is uniessential to that individual. It is modeled on Aristotle's essentialism in which the form or essence ofan individual is the principle of unity of that individual. For example, the form or essence of an artifact, like a house, is what unifies the material parts of the house into a new individual (over and above a sum of parts). Since an individual's gender is a social role (or set of social norms),the kind of unity in question is not the unity of material parts, as it is in the artifact example. Instead, the central claim of gender uniessentialism is that an individual's gender provides that individual with a principle of normative unity - a principle that orders and organizes all of thatindividual's other social roles. An important ingredient in gender uniessentialism concerns exactly which individuals are at issue - human organisms, persons, or social individuals? The Metaphysics of Gender argues that a social individual's gender is uniessential to it. Gender uniessentialism expresses the centrality of gender inour lived experiences and explores the social normativity of gender in a way that is useful for feminist theory and politics.