In this paper I provide a theory of the speech act of assertion according to which assertion is a species of joint action. In doing so I rely on a theory of joint action developed in more detail elsewhere. Here we need to distinguish between the genus, joint action, and an important species of joint action, namely, what I call joint epistemic action. In the case of the latter, but not necessarily the former, participating agents have epistemic goals, e.g., the acquisition of knowledge. It is joint epistemic action that assertion is a species of.