Not every speech act can be a lie. A good definition of lying should be able to draw the right distinctions between speech acts that can be lies and speech acts that under no circumstances are lies. This paper shows that no extant account of lying is able to draw the required distinctions. It argues that a definition of lying based on the notion of ‘assertoric commitment’ can succeed where other accounts have failed. Assertoric commitment is analysed in terms of two normative components: ‘accountability’ and ‘discursive responsibility’. The resulting definition of lying draws all the desired distinctions, providing an intensionally adequate analysis of the concept of lying.