 Modal discourse concerns alternative ways things can be, e.g., what might be true, what isn’t true but could have been, what should be done. This entry focuses on counterfactual modality which concerns what is not, but could or would have been. What if Martin Luther King had died when he was stabbed in 1958 (Byrne 2005: 1)? What if the Americas had never been colonized? What if I were to put that box over here and this one over there? These modes of thought and speech have been the subject of extensive study in philosophy, linguistics, psychology, artificial intelligence, history, and many other allied fields. These diverse investigations are united by the fact that counterfactual modality crops up at the center of foundational questions in these fields.