What does it mean for a legal norm to be valid? Does the validity of legal norms depend solely on their due enactment and social efficacy, or conceivably on their moral rightness as well? Does legal validity imply membership in the legal system, or perhaps applicability or observance within that system? In what way do we recognise that legal norms are valid? What are the structure and the layout of juristic argumentation concerning legal validity? Are the courts occasionally bound by valid positive law to apply invalid legal norms or provisions? These are some of the theoretical and practical questions that I aim to answer in this book.