Imprisonment of directors and employees for taking part in cartel activity is becoming an increasingly common penalty in western jurisdictions. Generally it is the only competition law offence that attracts a criminal sanction either as a matter of law or practice. This article examines the evidence in support of the alleged “harm done” by cartels, which it finds insubstantial, and refers by contrast to the limited academic literature available which suggests that generally cartels may be relatively ineffective and probably less damaging than other competition offences such as monopoly pricing or exclusionary behaviour.