Everyday language is saturated with appeals to what might be the case or to what must be true or to what cannot happen. Possibility, necessity, and impossibility are modal terms, and philosophers have long wondered how to best understand them. This volume traces the history of some of the most prominent and important contributions to our understanding of possibility and necessity and related concepts over the past two and half millennia of western philosophy, from ancient Greek philosophers through current debates in the 21st century.