The relationship between international law and municipal law has always been a fundamental feature of the study of international law as an academic discipline. Additionally, the practical interaction of the two normative regulatory systems presents tremendous consequences for the conduct of human affairs, nationally and internationally. An understanding of their interrelationship, interaction, and interplay (doctrinally and pragmatically) is essential for a scholarly appreciation of the nature, scope, and dimensions of the concept of universal jurisdiction. This chapter provides a comprehensive analysis of this relationship.