Although modern societies generally entrust enforcement of the criminal law to public prosecutors, most crimes in premodern societies were prosecuted privately. In classical Athens, ninth-century Germany, and England before the nineteenth century, there were no public prosecutors for most crimes. Instead, the victim or a relative initiated and litigated the cases. This article is the first rigorously quantitative analysis of private prosecution. It focuses on thirteenth-century England and uses statistical techniques, such as regression analysis, to show that changes in the treatment of settled cases can explain the rate of private prosecution.