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The two-step methodology for the identification of general principles of law deriving from domestic legal systems, consisting of a comparative analysis followed by a transposability test, seems accepted as the undisputed methodology in the current work of the International Law Commission on the topic. This article examines whether this two-step approach finds reflection in the practice of and before the PCIJ/ICJ and in international legal scholarship. The analysis finds that judicial practice does not entirely follow these two steps, but the method is widely upheld in doctrinal writing. The article argues that the decision to codify this two-step methodology can be viewed as progressive development by the Commission, and may signify the crystallization of this method of identification of general principles of law.

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.

The Appellate Body (AB) of the World Trade Organization (WTO) is currently under the concrete threat of ceasing its activities in the near future. This is the result of a series of unaddressed criticisms by some of the members of the organization, in particular the United States, regarding the alleged activist role that the AB has played. These criticisms involve a number of issues regarding interpretative practices developed by the organ. Against this backdrop, the present paper tackles the use of “relevant rules of international law” which are external to the WTO covered agreements, in particular through Article 31(3)(c) of the Vienna Convention on the Law of the Treaties, the provision codifying the so-called “principle of systemic integration”. It studies the reports in which reference to this provision was made and whether the use of non-WTO rules has been made in an expansive or limited way with respect to the mandate of the AB. It also assesses the reception of these reports by the WTO membership in the meetings of the Dispute Settlement Body. It is concluded that, while recourse to external sources under the VCLT provision could represent a potential source of expansion of the AB material applicable law in a way that could displease the WTO membership, the AB has transitioned from a progressive approach which marked the beginning of its functioning to a more narrow approach to the applicability of non-WTO sources in the more recent caselaw.