The idea that international law exists and operates as a system has become a quasiaxiomatic premise of our discipline. Yet why we think of international law as a system, what precisely makes it a system and the normative implications of calling it a system remain largely unaddressed or contested questions. This chapter unpacks the manner in which international lawyers use and mobilize the concept of system in their writings. It begins by attempting a short genealogy of the concept, looking at the historical and political context in which the concept emerged. It then examines the ontological split in the literature between the concept of system as fact and the concept of system as postulate. From there, it goes on to consider three prevailing conceptions of international law as a normative, a purposive and a cultural system.