Tradition is constantly at work in international legal thought and practice notwithstanding the attempt of international law to be nothing else than an autonomous discursive practice grounded in the actuality of the world. This chapter defines tradition, as the invocation of a pre-discursive past, that is a past that precedes the past of the discourse. It then discusses the systematic reliance on the idea of tradition in international law despite international law seeking to profile itself as a modern discourse grounded in the actuality of the world. Thereafter, this chapter turns more specifically to how tradition specifically plays out in relation to the sources of international law, and especially to the common opposition between traditional and non-traditional sources of international law.