Gaja, Giorgio Keywords: Ius cogens | Public international law | Vienna Convention on the Law of Treaties (Vienna, 23 May 1969) | State responsibility | Mots clefs: Jus cogens | Droit international public | Convention de Vienne sur le droit des traités (Vienne, 23 mai 1969) | Responsabilité des Etats | ABSTRACT Giorgio Gaja, Professor at the University of Florence, begins an analysis of the role of peremptory norms in contemporary treaty law. He devotes a first chapter of his course to treaty law. After some preliminary remarks on the relevance of…