This Article discusses natural law, the foundation of many international law principles. First, it describes the natural law and its bearing on the practice of international law. Second. it describes the concept of "the common good," a foundation of natural law. Third, it introduces the term "solidarity" and its relation to achieving the common good. Fourth, it describes the concept of "subsidiarity," a form of decision-making necessary for natural law to inform international law. Finally, it explains the suum cuique, a critical precept in natural law as it applies to international law. The Article concludes that natural law principles that rely on practical reason can make important substantive contributions to international law practice today.