The nature of authority has been much debated in legal and political philosophy, and the terminology is not stable. I shall begin by stipulating how I understand some of the key terms used in this paper1. I consider authority here as a property of laws and of legal systems. Authority must be distinguished primarily from power, efficacy, and validity. Authority is not the same as power. A person or institution may have the power to affect the way that people behave, but lack authority because he, she, or it is purely coercive. Authority is not the same thing as efficacy; efficacy simply means that there is conformity to the rules of the system, whether willing conformity or not. But it is possible for there to be conformity to the laws of a successful repressive system, and such a system would not have authority. Authority is not the same as validity. To be valid is to be pedigreed by the rule of recognition of the legal system. A legal rule may have that property, and yet lack authority because it is not a rule which there is any obligation to obey. If the law has authority, then the demands that the law makes of us are such that we ought to conform to them. Theorists sometimes distinguish between “legitimate authority” and “de facto authority”. It follows from the above that in my terminology “authority” means legitimate authority as that term has been typically understood, as authority which issues directives we have an obligation to obey. “Legitimate authority” is pleonastic.