The year 2020 marks the 75th anniversary of the United Nations Organisation, and the 50th anniversary of the United Nations Friendly Relations Declaration, which states the fundamental principles of the international legal order. In commemoration, some of the world's most prominent international law scholars from all continents have come together to offer a comprehensive study of the fundamental principles of international law. Each chapter in this volume reflects decades of experience, work and reflection by the most authoritative voices of the field. At the same time, the book is an invitation to end narrow specialisation and re-engage with the wider body of rules and processes that lie at the foundations of the international legal order.