This fully updated, go-to treatise explores the historical evolution and contemporary operation of international law in a time of increasing geopolitical uncertainty. It offers an accessible yet nuanced text for a variety of audiences, including university and law school students, practitioners, researchers, and others who want to know what international law is and what it does in the twenty-first century. Each chapter is followed by questions for discussion, as well as up-to-date suggestions for further reading. The new sixth edition weaves in-depth considerations of key cases, core disputes, and current events into a broad overview of all important aspects of the subject. Readers will find an authoritative discussion of traditional topics such as the sources of international law and the methods of international dispute resolution alongside consideration of pressing issues such as climate change, the advent of generative AI, and contemporary challenges to the domestic and international rule of law.