The category of mood is widely used in the description of languages and the formal analysis of their grammatical properties. It typically refers to features of a sentence’s form (or a class of sentences which share such features), either individual morphemes or grammatical patterns, which reflect how the sentence contributes to the modal meaning of a larger phrase or which indicates the type of fundamental pragmatic function it has in conversation. The first subtype, verbal mood, includes the categories of indicative and subjunctive subordinate clauses; the second sentence mood, encompasses declaratives, interrogatives, and imperatives. This work presents the essential background for understanding semantic theories of mood and discusses the most significant theories of both types. It evaluates those theories, compares them, draws connections between seemingly disparate approaches, and with the goal of drawing out their most important insights, it formalizes some of the literature’s most important ideas in new ways. Ultimately, this work shows that there are important connections between verbal mood and sentence mood which point the way towards a more general understanding of how mood works and its relation to other topics in linguistics, and it outlines the type of semantic and pragmatic theory which will make it possible to explain these relations.