Written by one of the world's leading international lawyers, this is a landmark publication in the teaching of international law. International law can be defined as 'the rules governing the legal relationship between nations and states'. However, with political, diplomatic and socio-economic factors shaping the law and its application, international law is much more complex. This refreshingly clear, concise textbook encourages students to view international law as a dynamic system of organising the world. Bringing international law back to its first principles and breathing new life and energy into the subject, the book is organised around four questions: Where does it come from? To whom does it apply? How does it resolve conflict? What does it say? This fourth edition includes references to new case-law and literature, and features (brief) discussions on recent topics of general interest, including the Ukraine invasion, global health law and energy law.