||Epistemic logics are logics that allow one to reason about knowledge in some way. Doxastic logics are similar, but allow one to reason about belief rather than knowledge. The languages of these logics contain knowledge or belief operators or predicates, governed by appropriate axioms or rules. (Just what axioms and rules are appropriate is a controversial matter, however.) Many epistemic and doxastic logics are modal logics, whose languages contains one or more knowledge or belief operators, and whose semantics is given in terms of relational Kripke models. In such models, the points or states can be thought of as epistemically (or doxastically) possible worlds, related to one another by epistemic (or doxastic) accessibility relations. This modal approach to epistemic and doxastic logic has been widely adopted in formal logic, philosophy, computer science, artificial intelligence, economics and game theory. The category doxastic logic also includes work on belief revision and belief update, which addresses the question: how should an agent revise or update its beliefs, on receiving conflicting information?