The decision whether to challenge an arbitrator is among the most sensitive decisions that counsel can make in the course of an arbitration.The consequences of doing so, or not doing so, can be serious for both parties and counsel.To study the success rates and consequences of disqualification challenges, the authors gathered data on such decisions from across four arbitral fora: the International Centre for Settlement of Investment Disputes (“ICSID”); the Permanent Court of Arbitration (“PCA”); the United Nations Commission on International Trade Law (“UNCITRAL”); and the London Court of International Arbitration (“LCIA”).We find that while successful challenges against arbitrators are infrequent across all of these fora, there are nonetheless significant differences in success rates across the fora.We also find that while disqualification challenges are generally decided promptly, there are longer delays in ICSID proceedings.