One fundamental principle of investor-State arbitration is the ability of parties to have their disputes resolved by independent and impartial arbitrators. In order to ensure adherence to this principle, laws and rules governing investor-State arbitrations grant parties the right to challenge arbitrators lacking these qualifications. Given the importance of ensuring arbitrator independence and impartiality, and as evidenced by the recent decision in and subsequent commentary on Vivendi v. Argentina, practitioners and stakeholders in investor-State arbitrations are devoting significant efforts to defining the substantive standards warranting arbitrator challenges, and to understanding what those standards require in practice.