This chapter examines how international relations (IR) scholarship has approached two central questions concerning international law and legalisation: why do states create international law, and what makes a particular norm ‘legal’ in nature? It then outlines the concept of legalisation as described in Abbott et al.’s well-known article of the same name. Under the classic legalisation framework, legalisation has three components: obligation, precision and delegation. The chapter argues that the classic OPD framework cannot fully capture the expanding role of non-state actors or conceptualise law as a process. It therefore proposes an adapted model for the transnational legal system that incorporates a crucial omitted dimension – implementation. Implementation refers to the concrete actions taken by agents to translate legal or law-like principles into practical, workable instructions for courts, governments, companies, and other non-state actors.