 A variety of norms, social and linguistic practices, agent interactions and games presuppose common knowledge, first formalized by Aumann (1976) and with earliest epistemic logical treatments by Lehmann (1984) and by Halpern and Moses (1984). In order to see how epistemic logic sheds light on these phenomena, it is necessary to introduce a little more formalism. Following the standard treatment (see, e.g., Fagin et al. 1995), we can syntactically augment the language of propositional logic with n knowledge operators, one for each agent involved in the group of agents under consideration. The primary difference between the semantics given for a mono-agent and a multi-agent semantics is roughly that n accessibility relations are introduced. A modal system for n agents is obtained by joining together n modal logics where for simplicity it may be assumed that the agents are homogenous in the sense that they may all be described by the same logical system. An epistemic logic for n agents consists of n copies of a certain modal logic. In such an extended epistemic logic it is possible to express that some agent in the group knows a certain fact that an agent knows that another agent knows a fact etc. It is possible to develop the logic even further: Not only may an agent know that another agent knows a fact, but they may all know this fact simultaneously.