Ever since Hart¿s The Concept of Law , legal philosophers agree that the practice of law-applying officials is a fundamental aspect of law. Yet there is a huge disagreement on the nature of this practice. Is it a conventional practice? Is it like the practice that takes place, more generally, when there is a social rule in a group? Does it share the nature of collective intentional action? The book explores the main responses to these questions, and claims that they fail on two main counts: current theories do not explain officials¿ beliefs that they are under a duty qua members of an institution, and they