H.L.A. Hart was the foremost Anglophone philosopher of law in the twentieth century, and he was rivaled only by Hans Kelsen as the foremost philosopher of law in any language during that century. Among his many sterling accomplishments in the philosophy of law was his reinvigoration of the tradition of legal positivism. His revival of that tradition greatly strengthened it by transforming it in some major respects – not least by severing it from the command theory of law that had been propounded (in distinct versions) by his illustrious predecessors Jeremy Bentham and John Austin. Yet, in spite of the distance that Hart put between himself and those predecessors, he was firmly aligned with them in his emphasis on the distinction between what legal institutions are and what legal institutions morally ought to be. With his general insistence on the separability of law and morality, he established himself as an opponent of natural-law theorists and their efforts to show that law is an inherently moral phenomenon. Indeed, the arguments through which he impugned a multiplicity of natural-law lines of reasoning are one feature of his book THE CONCEPT OF LAW that has cemented its place as a classic text with which generations of legal philosophers will perennially grapple. The present essay, written for the CAMBRIDGE COMPANION TO LEGAL POSITIVISM, provides an overview of Hart's contribution to the tradition of positivist thinking about law.