The language of law includes normative or prescriptive terms such as 'obligation' and 'permission'. How do we explain the meaning of prescriptive legal language? This has long been regarded as a problem for positivists, since at first glance their view suggests we can derive an ought – a legal obligation or right or permission – from descriptive social facts alone. This Element outlines what we should want from a semantics of prescriptive legal language, critically evaluates four leading semantic accounts, and argues that legal prescriptivity is not, in the end, a problem for positivists.