This paper assesses BitTorrent's robustness against selfish peers who try to download content faster than their fair share by abusing existing protocol mechanisms. We present three exploits that can deliver potential benefits to a selfish peer and evaluate their impact on both public and private download sessions. Our results show that BitTorrent is quite robust against these exploits. Although selfish peers can sometimes attain high download throughput and compliant peers' download rates suffer slightly in consequence, we observe no significant degradation of the overall system's quality of service. We identify scenarios where a selfish peer could attain significant benefits at the expense of compliant peers, and discuss the protocol characteristics that render these scenarios unlikely and hence lead to the system's robustness. Copyright © 2007 John Wiley & Sons, Ltd.