The Articles on State Responsibility for Internationally Wrongful Acts (‘ARSIWA’) constitute an experiment in international law-making. Unlike other successful projects of the International Law Commission (‘ILC’), such as its work on the law of treaties and diplomatic and consular relations, the ARSIWA have not yet led to the adoption of a multilateral treaty. Yet, their text is cited commonly as the authoritative statement of the law on state responsibility with investment tribunals being by far their most frequent users. This well-recorded paradox calls for a reflection on the ways in which investment tribunals make use of the ARSIWA. This chapter examines the methods which investment tribunals explicitly or implicitly employ when using the ARSIWA in order to determine the content of rules of general international law on state responsibility. The chapter then proceeds to critically assess these findings from two perspectives: the overarching aims of the law of state responsibility and the doctrine of sources of international law. The chapter synthesises these empirical and doctrinal insights into a proposal for a common framework for the use of ARSIWA.