Systems approaches were an early part of the development of norms research as a subfield of International Relations but have been eclipsed by approaches which emphasise the role of actors, processes, and relationships. However, with new scholarship beginning to explore complex interactions of different norms and their relationship to the structure of the international system, the time to reassess existing systemic theories of international norms is now. This chapter traces the use of different types of systemic norms theory, including the norm life cycle, norms-as-structure, biological and ecological understandings of norm interaction and evolution, and complex systems theory (including regime complexes). By understanding international norms as emergent properties of a complex international system, we focus scholars’ attention on how the international system itself can affect the emergence, diffusion, contestation, and evolution of international norms and vice versa. The chapter finishes by employing a systems approach to understanding norm challenges regarding the rule of law.