Identification of and Resort to Customary International Law by the WTO Appellate Body

The recourse to customary rules by international adjudicators is a valuable tool for the understanding of this source of international law. This chapter examines patterns employed by the WTO’s Appellate Body when referring to customary international law. This forum is of interest due to its limited material jurisdictional, as it can only ascertain violations of WTO law. The chapter thus analyses the means through which the adjudicators identify customary rules, which customary rules they resort to and for which purposes they rely on these ‘extraneous’ sources. Given these jurisdictional limitations, it is also relevant to understand the difference between the use of customary rules for purposes of interpretation and the application of these sources within WTO dispute settlement. The Appellate Body’s reliance on customary international law is limited: it only resorts to codified rules of treaty interpretation and state responsibility. It also does not properly identify rules of international law, but asserts their existence through other authoritative references. Through this methodology, and by only referring to rules on treaty interpretation and state responsibility as customary law, the AB has more leeway in not overstepping the line between application and interpretation of customary international law and by consequence its jurisdictional limitations.