Lecture summary to print: 20 years later - How has International Law evolved as a legal Order? In 2000, Professor Dupuy delivered the General Course of International Law at the Hague Academy of International Law in The Hague, under the title L’Unité de l’ordre juridique international; it is an argumented criticism of the theory of the so called «fragmentation» of international law. Pierre-Marie Dupuy developed in this course, published in 2003 (Vol. 297) his theory on the dialectic tension between two grounds of unity of the international legal order: on the one hand the formal one, based on the technical forms and procedures of law, directly connected with the Hart’s theory of law as developed in The Concept of Law; on the other hand, the substantial unity, a hierarchical one, based not any more on the forms, which are neutral in essence, but on the very content of some peremptory norms of international law (jus cogens); this substantial unity entails a number of rather easily identifiable rules including in particular some fundamental principles of human rights law; but, as demonstrated by the international case law, substantial unity also includes a few «fundamental» rights of the State, to speak like the ICJ in one of its advisory opinions (Legality of the Threat or Use of Nuclear Weapons, 1996) and this constitutes a supplementary factor of inherent tension. Two competing principles of unity, formal and substantial or material) for the same legal order are not necessarily the best means for ensuring the persistence and overall stability of the whole system! One would have probably been more efficient. But so it is (or was since the sixties) because of the common will of the States composing what is called «the international community of states as a whole» (Art. 51 VCLT), a term which is to be taken not as a mere description of political reality but as the recourse to what is called in legal technics «a legal fiction» (fiction juridique). In his general course, far from predicting the final supremacy of one of these grounds of unity over the other, the two of them being at the same time competing and complementary, the author privileged an open ended conclusion which is far from an Hegelian synthesis; rather, it is indicative of the impossibility of drastically forecasting the overall evolution of the whole international legal system; this is because international law is not only an abstract (and formal) combination of primary and secondary rules; it is also directly conditioned by a number of structural and/or contextual sociopolitical elements which are in constant evolution (a reason why the classical positivist doctrine does not want to take them into account). Whatever the case may be, the tension between the two sources of unity of the international legal order is at the very core of the inherent dynamic of this order. Almost twenty years after the delivery of this course, it seems interesting to review and reconsider this theory of the two kinds of unity of the international legal order (formal and substantial) in particular at a time when an increasing number of «populist» leaders very much seem to ignore, or voluntarily deny the validity of some of the key substantial principles on which the international legal system was re-founded within and around the United Nations in 1945; is, in this respect, the future of international law as a legal order more predictable than before or not? And, if yes, in which direction?