In this paper I provide a new account of linguistic presuppositions, on which they are ancillary speech acts defined by constitutive norms. After providing an initial intuitive characterization of the phenomenon, I present a normative speech act account of presupposition in parallel with Williamson’s analogous account of assertion. I explain how it deals well with the problem of informative presuppositions, and how it relates to accounts for the Triggering and Projection Problems for presuppositions. I conclude with a brief discussion of (...) the consequences of the proposal for the adequacy of Williamson’s account of assertion. (shrink)