The abundance and importance of temporary project teams in society introduces the need of understanding their nature. The purpose of this article thus is to highlight the existence of an only accidentally investigated type of team that we identify as Impromptu teams, and analyze their role in a temporary organization. Based on a detailed retrospective account of the infamous disaster on Mount Everest in 1996, we identify three examples of Impromptu teams. The three examples indicate that the teams are characterized by being triggered by an unexpected event, and formed through a bottom-up process, where joining the team is voluntary and the activities are based on a logic of appropriateness, rather than rule following. The identification and nature of Impromptu teams have implications far beyond Mount Everest, since most organizations at some point need to use teams similar to the identified examples.