 This entry is about worlds that are not possible, with “possible” understood in an unrestricted sense. Start with the intuitive idea of the totality of possible worlds, which capture all and only the genuine possibilities. The worlds we are interested in are not in there. These worlds are often called logically impossible worlds, as logical laws such as the Law of Non-Contradiction or the Law of Excluded Middle are assumed to be the most general and topic-neutral: they are supposed to hold at all possible worlds. From now on, we are talking of impossible worlds simpliciter, meaning worlds that are not possible with respect to an unrestricted notion of possibility, however this is further characterized.