This is a book about semantic theories of modality. Its main goal is to explain and evaluate important contemporary theories within linguistics and to discuss a wide range of linguistic phenomena from the perspective of these theories. The introduction describes the variety of grammatical phenomena associated with modality, explaining why modal verbs, adjectives, and adverbs represent the core phenomena. Chapters are then devoted to the possible worlds semantics for modality developed in modal logic; current theories of modal semantics within linguistics; and the most important empirical areas of research. The author concludes by discussing the relation between modality and other topics, especially tense, aspect, mood, and discourse meaning. Paul Portner's accessible guide to this key area of current research will be welcomed by students of linguistics at graduate level and above, as well as by researchers in philosophy, computational science, and related fields.