Questions about sincerity play a central role in our lives. But what makes an assertion insincere? In this paper we argue that the answer to this question is not as straightforward as it has sometimes been taken to be. Until recently the dominant answer has been that a speaker makes an insincere assertion if and only if he does not believe the proposition asserted. There are, however, persuasive counterexamples to this simple account. It has been proposed instead that an insincere assertion that p is one made by a speaker who (a) does not express his belief that p; or (b) does not believe that he believes that p; (c) does not assent to p; or (d) does not express any of these cognitive states. We show that these alternative accounts also face counterexamples.