International organisations are inherently purposive actors within the international legal system, created and empowered by States to pursue finite common objectives. This teleological dimension has come to play a prominent role in the way in which international law rationalises international organisations, with their purposes given a significant, often determinative, role in delimiting their competences. This article argues that this is the product of a conscious shift in legal reasoning that took place in the aftermath of World War II. Through an analysis of a series of key post-War decisions, it identifies the common features of this ‘teleological turn’ and, disentangling it from other forms of legal reasoning, examines its unique underlying logic and normative claims. It demonstrates that while the teleological turn offers prospects for the systemic development of international governance, an increasingly abstract approach to the concept and identification of an organisation's ‘purpose’ raises a number of unresolved questions which cast a shadow of indeterminacy over the law of international organisations.