In some languages, every declarative sentence includes a morpheme specifying the speaker's evidence or source of information. This article provides an overview of the central theoretical questions addressed in recent research on Evidential morphemes. First, I discuss the question of whether Evidentials constitute a coherent closed-class system, independent of other systems of grammar. Next, I briefly consider the evidence for an Evidential head in the syntactic representation. Finally, I review the ways in which Evidentials resemble and differ from epistemic modals.