 Epistemologists are interested in the norms governing the structure and dynamics of systems of belief: how an individual's beliefs must cohere in order to be considered rational; how they must be reflected in decision making; and how they ought to accommodate new evidence. Formal epistemologists pursue these questions by constructing mathematical models, or “formal representations,” of belief systems that are, in some sense, epistemically exemplary. Broadly speaking, these models capture something important about how an ideally rational agent would manage her epistemic life. This entry gives an overview of the formal representations that have been proposed for this purpose.