Despite the undeniable ‘treatification’ of international investment law, that should not lead one to the erroneous assumption that, as a direct consequence of this, other sources of law and custom in particular, become gradually and increasingly more irrelevant in this particular filed of international law. Contrarily, customary rules remain of fundamental importance in what has been called ‘the age of treatification of international investment law’. Furthermore, custom, both in its primary rule and secondary rule incarnation, potentially will even further grow in its importance to international investment law and arbitration, considering the seemingly increasing trend of what may be dubbed if not a ‘de-treatification’ then at least a decrease in the number of treaties (or treaty ratifications) relating to international investment law. Even with the justified criticisms about false narratives and the creation of custom being a reflection of prior and current power structures, the study of custom and its function across all stages of its life-cycle has a lot to yield. Both these criticisms and the general academic inquiries into the lacunae of custom (at the identification as well as the interpretation/content-determination stages) contribute to the gradual refinement of our understanding of how custom works, how it is used, what gaps it has and how it can adapt to modern challenges and new circumstances.