Introduction: Stories about International Organisations, Non-State Actors and the Formation of Customary International Law - Sufyan Droubi and Jean d'Aspremont 1 International Organisations as Autonomous Actors - Nikolaos Voulgaris 2 Capturing the Juridical Will - Catherine Brölmann 3 Opinio Juris : Between Mental States and Institutional Objects - Sufyan Droubi 4 Beyond the Either-Or Paradigm - Lorenzo Gasbarri 5 Between Anthropomorphism and Artificial Unity - Maiko Meguro 6 Custom as the product of successful argumentative campaigns - Fabian Cardenas 7 Unlearning some common tropes - Jean d'Aspremont 8 International Organizations and non-State actors in the International Law Commission's Conclusions on Identification of Customary International law - Michael Wood 9 The International Law Commission Perspective on Non-State Actors and Customary International Law - Veronika Bílková 10 The International Law Commission in the Eyes of the International Law Commission: Mirror or Looking-glass? - Luíza Leão Soares Pereira 11 The Riddle of Custom: General Assembly Resolutions - Rossana Deplano 12 The United Nations Secretariat and Custom - Jean-Baptiste Merlin 13 The Making of Custom through Sanctions of International Organisations - Marusa T. Veber 14 The International Committee of the Red Cross and Custom - Iris Müller 15 Treaty Bodies and States: Shaping Custom - Kasey McCall-Smith 16 The Contribution to Customary International Law by Territories under International Administration - William Thomas Worster 17 The formation of customary international law by de facto regimes - Antal Berkes 18 Investors in the Formation of Customary International Law - Tomoko Yamashita 19 Adjudicators, Guardians, and Enforcers: Taking the Role of Non-Governmental Organisations in Customary International Law-Making Seriously - Valentina Azarova Index.