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? After distinguishing between essentialism about gender viewed as a kind and essentialism about gender in relation to individuals and their lived experiences, 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 of an 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 that individual's other social roles. An important ingredient in gender uniessentialism concerns exactly which individuals are at issue -- human organisms, persons or social individuals? Given the view of gender as a social role the claim of gender essentialism can only be coherently expressed of social individuals. The Metaphysics of Gender argues that a social individual's gender is uniessential to it.