Forty years after his death, Hans Kelsen (1881-1973) remains one of the most discussed and influential legal philosophers of our time. This collection of new essays takes Kelsen's Pure Theory of Law as a stimulus, aiming to move forward the debate on several central issues in contemporary jurisprudence. The essays in Part I address legal validity, the normativity of law, and Kelsen's famous but puzzling idea of a legal system's 'basic norm'. Part II engages with the difficult issues raised by the social realities of law and the actual practices of legal officials. Part III focuses on conceptual features of legal systems and the logical structure of legal norms. All the essays were written for this volume by internationally renowned scholars from seven countries. Also included, in English translation, is an important polemical essay by Kelsen himself.