Having distinguished to his satisfaction mind and body so successfully, Descartes faced the challenge of sticking them back together so as to explain how they constitute a single thing, the whole human being. To avoid the charge of Platonism, he asserts that the union is not an accidental unity of two substances, not a “pilot in a ship”, but a “substantial union” (unio substantialis) about which he tries to say as little as possible. Some commentators think the union consists in nothing more than the interaction of mind and body; others think it constitutes a third kind of (composite) substance besides mind and body. I argue that neither of these interpretations makes sense of the texts and appeal to the medieval traditions in which the term ‘unio substantialis’ appears to argue that the term is ambiguous and that Descartes seems rather deliberately to be exploiting this fact. The discussion of the union is, however, one reason to think that Descartes’ ontology includes more than substances and their modes and attributes. There are composites which are neither substances in themselves (although they are composed of them) nor mere aggregates of substances.