In fact, law is necessarily very vague. So if vagueness is a problem for legal theory, it is a serious problem. The problem has to do with the ideal of the rule of law and with the very idea of law: if vague standards provide no guidance in some cases, how can the life of a community be ruled by law? The problem has long concerned philosophers of law; the papers at this symposium address it afresh by asking what legal theory may have to learn from (or contribute to) work on vagueness in philosophy of language and philosophy of logic. Here I will not try to state the implications of vagueness for philosophy of law; I will try to set the stage by showing that vagueness is both an important and an unavoidable feature of law.