The word 'ought' is one of the core normative terms, but it is also a modal word. This book 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. The inferentialist theory defended in this book agrees with descriptivist theories in metaethics that specifying the truth conditions of normative sentences is a central part of the explanation of their meaning.