According to the increasingly popular knowledge account, assertion is governed by the rule that speech acts of that kind require knowledge of their content. Timothy Williamson has argued that this knowledge rule is the constitutive rule of assertion. It is argued here that it is not the constitutive rule of assertion in any sense of the term, as it governs only some assertions rather than all of them. A (qualified) knowledge rule can in fact be derived from the traditional analysis of assertion according to which assertion is the linguistic expression of belief. Because it is more informative, this analysis provides a better point of departure for defending the knowledge account than Williamson’s view according to which the knowledge rule is part of the analysis of assertion.