Most contemporary observers of international investment law will likely share with the author of these lines the impression that the criticism expressed by many commentators over the last decade with respect to the operation of investor–State arbitration is no longer of merely theoretical interest.2 Practice, broadly understood so as to encompass more circumspect stances taken by arbitration tribunals, more carefully reflected treaty negotiation and design, and even the wide variety of events, discussions and writings through which a certain idea of the state of investment arbitration is formed, is indeed changing. And the drivers are not confined to stigmatized countries (and authors) but also include an increasing number of developed countries and even the European Commission. In academic and practitioner circles, the initial polarization between pro-investor and pro-State views is no longer a useful discourse, if it ever was. This is not—and should not be—the time to point fingers at people for rapacious or hypocritical practices or to do entirely away with a system of investment arbitration that, despite its many flaws, also has many advantages. Rather, the time is ripe to formulate specific suggestions as to how to adjust the international legal framework relating to investment transactions. It is in this context that the concept of investor diligence in investment arbitration is starting to be discussed in more detail, not just as a matter of activism but as one of law—positive law.