The chapter scrutinizes the recourse to legal witnesses on points of international law through the lens of the specific texture of customary international law. Part I will present and comparatively assess the abundance of the recourse in investment arbitration to expert witnesses on issues of international law; part II will then proceed to a theoretical analysis which will set, according to a formalist approach of that source of law, and test the hypothesis that international law witnesses in investment arbitration could well be justified when they deal with customary international law. It will conclude that, at most, customary norms may have been the Trojan horse of the recourse to international law experts in investment arbitration since international law witnesses are seldom relied on for the purposes of ascertaining the contents of customary international law. Once the relationship between customary law and expert witnesses will be discarded, Part III will examine an alternative justification which has more to do with the sociology of investment law and with its constant search for legitimacy, than with any formal analysis of the sources of international law.