Building on his contributions to institutional legal theory in Institutional Legal Facts of 1993 (Law and Philosophy Library, volume 18), the author presents a comprehensive theory of legal institutions. To that end, the initial theoretical approach, which mainly concentrated on problems connected with legal powers and legal acts (acts-in-law), is widened to allow for the development of a theory of legal judgements capable of accounting not only for enacted but also unwritten law (legal principles and customary law). With the use of the concept of institutional legal facts, the structure of legal institutions is analyzed in detail. In addition to that, a classification of legal institutions is provided. Extensive attention is given to logical, as well as doctrinal problems connected with a conception of legal validity as the mode of existence of legal conditions rather than as a value of legal norms similar to the truth of propositions. The study results in an elaborate conceptual framework for institutional analysis of positive law. In a final chapter the analytical potential of the framework is put to the test by applying it to the branch of public international law known as the `law of treaties'. Readership: Specialists in legal theory and lawyers interested in theoretical issues, particularly in linguistic approaches and questions related to the institutional nature of law.