This chapter examines the interplay between Philip Jessup’s experience of the early years of the United Nations and the development of his scholarship, in particular his inclusive and pragmatic conception of ‘transnational law’. It is contended that Jessup’s UN experience informed writings that evince a practitioner’s feel for the role of diplomacy in the workings of international law, as well as aliveness to the possibilities of novel institutional forms and processes. The chapter goes on to apply Jessup’s scholarly insights to contemporary challenges of the UN, focusing on the Sustainable Development Goals. The achievement of the SDGs emerges as a thoroughly transnational challenge, requiring application and coordination of each of the strands of Jessup’s famous definition of ‘transnational law’.