The Common Concern of Humankind today is central to efforts to bring about enhanced international cooperation in fields including, but not limited to, climate change. This book explores the expression's potential as a future legal principle. It sets out the origins of Common Concern, its differences to other common interest legal principles, and expounds the potential normative structure and effects of the principle, applying an approach of carrots and sticks in realizing goals defined as a Common Concern. Individual chapters test the principle in different legal fields, including climate technology diffusion, marine plastic pollution, human rights enforcement, economic inequality, migration, and monetary and financial stability. They confirm that basic obligations under the principle of 'Common Concern of Humankind' comprise not only that of international cooperation and duties to negotiate, but also of unilateral duties to act to enhance the potential of public international law to produce appropriate public goods.