‘Where the ILC stopped in its 2018 Conclusions on Identification of Customary International Law, this book continues. It offers a rich variety of theoretical and practice-inspired reflections, doing justice to the ways in which international organizations and non-state actors contribute to the development of customary international law.’—Niels Blokker, Professor emeritus of International Institutional Law (Schermers Chair), Leiden University'Overall, the book is a great source for anyone interested in the general theoretical issues of customary international law, as well as those who have an interest in narrower questions on the role that international organisations and non-state actors have at different stages of the making of customary international law.' —Journal of International Law of Peace and Armed Conflict'The book is a great source for anyone interested in the general theoretical issues of customary international law, as well as those who have an interest in narrower questions on the role that international organisations and non-state actors have at different stages of the making of customary international law. Outside of theory, the volume offers the reader an opportunity to look at the theoretical issues from a practical perspective.' —Humanitäres Völkerrecht