In The Critique of Pure Reason, Kant claims that from the time of Aristotle until his time, logic had been “unable to take a single step forward, and therefore seems to all appearance to be finished and complete.”1 Moreover, he adds that some alleged improvements were only minor changes or, even worse, confusing and full of misunderstandings. But contrary to this vision, the last two centuries have witnessed an extraordinary rebirth of logic. New approaches to classical problems, as well as new horizons opened to logical exploration (e.g., modal logic, the logic of relevance, the logic of action), have gained a legitimate reputation in contemporary philosophy. One of these new logical domains is deontic logic, the branch of logic that offers a formal analysis of normative discourse. Law is one of the most important normative fields, and deontic logic constitutes an invaluable aid for legal scholars and philosophers in the analysis of fundamental legal concepts. More specifically – as we try to show – deontic logic can be regarded as an essential tool to understand both the systematic structure of law and its dynamic nature. Undoubtedly, deontic logic is also useful for the evaluation of moral discourse, but in this book we limit our attention to the legal domain, with very few and merely incidental remarks on morality.