epistemic logic has evolved from the initial philosophical to find diverse applications in computer science as a means of reasoning about knowledge and belief of agents. The authors begin by presenting the necessary apparatus from mathematics and logic, including Kripke semantics and logic of the famous capital K, T, S4 and S5. Then they turned to the application in the context of distributed systems and artificial intelligence: topics covered include the notion of common knowledge, distributed knowledge, explicit and implicit belief, the interplay between knowledge and time, and knowledge and action, and the graded (or numerical) variation epistemic operator.