The word 'ought' is one of the core normative terms, but it is also a modal word. In this book Matthew Chrisman develops a careful account of the semantics of 'ought' as a modal operator, and uses this to motivate a novel inferentialist account of why ought-sentences have the meaning that they have. This is a metanormative account that agrees with traditional descriptivist theories in metaethics that specifying the truth-conditions of normative sentences is a central part of the explanation of their meaning. But Chrisman argues that this leaves important metasemantic questions about what it is in virtue of which ought-sentences have the meanings that they have unanswered. His appeal to inferentialism aims to provide a viable anti-descriptivist but also anti-expressivist answer to these questions.