The disparity of interpretation of the Fair an Equitable Treatment Standard in investor-state arbitration and the subsequent lack of uniformity in the criteria used for attributing compensation for its beaches call for the need to clarify its scope and components. It is true that international investment law is a highly fragmented field lacking a multilateral treaty source. It is also true that the proliferation of international arbitration tribunals and Bilateral Investment Treaties (BITs) conveys there is no single applicable law, no binding case law or stare decisis, no uniformity in the composition of the tribunals, and no overarching institutional guidance. Yet, despite this undeniable fragmentation, international investment law can rely upon general principles of law, general international law, international economic law and customary international law to help clarifying the obligations the FET standard imposes upon states and the subsequent rights it grants to investors. This article undertakes the task of setting a framework for the interpretation and application of the FET standard that helps improving legal certainty in the field of international investment arbitration. Based on the study of 55 decisions of the ICSID and a purposive interpretation of international investment law, I argue that the FET standard ought to be interpreted within a Rule of Law framework. This should allow for better clarity about the obligations and rights it conveys, without curtailing its significance in judicial reasoning as a notion à contenue variable.