 Epistemic logic is a subfield of epistemology concerned with logical approaches to knowledge, belief and related notions. Though any logic with an epistemic interpretation may be called an epistemic logic, the most widespread type of epistemic logics in use at present are modal logics. Knowledge and belief are represented via the modal operators K and B, often with a subscript indicating the agent that holds the attitude. Formulas \(K_{a}\varphi\) and \(B_{a}\varphi\) are then read “agent a knows that phi” and “agent a believes that phi”, respectively. Epistemic logic allows the formal exploration of the implications of epistemic principles. For example, the formula \(K_{a}\varphi\rightarrow\varphi\) states that what is known is true, while \(K_{a}\varphi\rightarrow K_{a}K_{a}\varphi\) states that what is known is known to be known. The semantics of epistemic logic are typically given in terms of possible worlds via Kripke models such that the formula \(K_{a}\varphi\) is read to assert that \(\varphi\) is true in all worlds agent a considers epistemically possible relative to its current information. The central problems that have concerned epistemic logicians include, for example, determining which epistemic principles are most appropriate for characterizing knowledge and belief, the logical relations between different conceptions of knowledge and belief, and the epistemic features of groups of agents. Beyond philosophy proper, epistemic logic flourishes in theoretical computer science, economics, and related fields.