This book presents a comprehensive defence of legal positivism on the basis of a novel account of social conventions. Marmor argues that the law is founded on constitutive conventions, and that consequently moral values cannot determine what the law is. On the basis of a theory of social conventions and an analysis of law's authoritative nature, the book sets out the scope of law in relation to moral and other critical values. The book also maintains, however, that moral values are objective. It comprises a detailed analysis of the concept of objectivity, arguing that many aspects of the law, and of moral values, are metaphysically objective.