This dissertation places cooperative action at the core of a systematic account of the linguistic, epistemic, ethical, and social dimensions of our declarative practices—of the ways we communicate to each other about the world. Say you and I are talking about that time Sam was detained and suddenly released by the police, and I assert “He bribed them.” I begin by arguing that my act is best understood as the act of cooperatively offering information to achieve the shared goals that structure our conversation. This means that, if I was only guessing and Sam did not actually bribe the police, my failure is an instrumental failure to achieve what I committed to do with you—just as passing you baking soda instead of salt is a failure in our joint project of making pasta. Crucially, unlike other views in the literature, this view neatly explains the distinctively second-personal entitlement you have to reproach me for my failure; and it avoids the problems of accounts that explain my failure in terms of the evolutionary function of assertion. Moreover, the view I develop also explains why falsely insinuating that Sam bribed the police by merely saying “he offered them hundreds of reasons to let him go” is often taken to be less committal, and thus less morally wrong, than my assertion above. The normative distinction between committal and non-committal speech is best captured by the distinction between cooperative and uncooperative speech—as opposed to the distinction between literal and non-literal speech, as is widely assumed in the literature. And, finally, I argue that the relationship between knowledge and successful action gives us reason to think that assertion requires something at least close to knowing what we say, and not, for instance, rationally believing it, or merely saying true things.