Extreme case formulations (ECFs) are semantically extreme formulations that invoke the maximal or minimal properties of events or objects, such as “everyone,” “nobody,” “always,” “never,” “completely,” “nothing,” and so on. This entry reviews seminal work on ECFs, identifying their key features and interactional uses. These include their uses in defending against or countering challenges to the legitimacy of complaints, accusations, justifications and defenses; in proposing the objective (rather than circumstantial) nature of a phenomenon; in proposing that some behavior is right or wrong by virtue of being widespread; and in producing designedly non-literal (and thus not accountably accurate) descriptions that display various kinds of investment on the part of speaker, and can also be used in actions such as joking, teasing and irony. In addition, the entry describes some of the applied interactional research in which ECFs have been shown to be an important resource for participants, and thus for analysts.