This chapter begins by outlining an alternate, ‘interactional’, understanding of the concept of ‘sources of law’, which it takes to refer to processes that are shaped by requirements of legality and through which legal norms are made and remade. This approach does not entail that the law-making methods listed in Article 38 of the International Court of Justice (ICJ) Statute have ceased to matter in international environmental law (IEL)—far from it. The interactional law framework takes seriously what international actors do. The chapter, therefore, explores the law-making processes listed in Article 38 in turn, and then moves on to consider newer processes. The interactional framework and its practice-based understanding of legality illuminate the existence of resilient and relatively stable law-making processes as well as the emergence of new law-making processes.