Although the World Trade Organization (WTO) may today be the international organization most associated with the role of overseeing the world economy, 90 years ago, that role was held by the International Labour Organization (ILO). The ILO not only showed how a functional international organization could help states cooperate on economic policy, but it also showed the utility of new principles in international law for disciplining domestic policy. Students of the WTO may not realize how important the ILO experience was in shaping the trading system. This chapter explains how international trade law in the WTO has been directly influenced by international labour law. The inter-organizational learning extends over matters of substantive law as well over a judicially-based dispute settlement system. For substantive law, the most prominent example is the trade law on special and differential treatment for developing countries. Such special treatment harks back to the ILO Constitution of 1919 which was proclaimed the Treaty of Versailles. WTO law language on full employment and social development is also traceable to ILO norms, particularly the Declaration of Philadelphia of 1944. A more remarkable borrowing occurs in WTO dispute settlement which is premised on numerous principles and procedures that were first articulated for the ILO. Ironically, these innovative procedures – such as adjudication for complaints, trade sanctions, compliance review, and the omission of the remedy of reparation – were never actually used by the ILO because that tripartite body preferred to promote higher standards by persuasive techniques.