Challenges to investment arbitrators are increasingly common. Using data from different arbitral institutions up to 2019, this chapter traces a remarkable upsurge in the number of arbitrator challenges from 2010 to the present. On the one hand, many challenges may be of a purely tactical character, designed by the parties – typically the respondent state – to delay proceedings or pressure a party to settle or withdraw a complaint. On the other hand, many arbitrators may be legitimately vulnerable to challenges; and the increase in number may suggest that the system is (or should be) taking more seriously concerns such as repeat appointments by the same party, double hatting, and issue conflicts. The author argues that arbitrator challenges may contribute to the legitimacy of the adjudicative process. They ensure that independence and impartiality is maintained in practice and signal to prospective arbitrators and their appointers the risks of non-disclosure or certain types of appointments.