This chapter examines the unparalleled influence that the Court’s decisions have had on the Commission’s codification and progressive development of areas of the law under its consideration. It illustrates not only the great extent to which many of the Commission’s propositions have borrowed their authority from the pronouncements of the Court, but also the significant impact of the latter on the Commission’s choices concerning terminology and programme of work. The chapter further demonstrates the Commission’s conscious efforts to support the Court’s cause more broadly, including by encouraging the expansion of the Court’s jurisdiction and by promoting the doctrine of the sources of international law enshrined in its Statute.