Abstract The legal “puzzle” raised by modern blackmail is that although it is lawful to disseminate harmful information about another person, just so long as the information is true, it is unlawful to extort money by making threats to do so. Roman law took a different approach. It was unlawful to reveal the harmful information unless the speaker could show a privilege to speak, usually that the public interest would be served by the revelation. For this reason, it was unlawful to threaten to do so unless such a privilege existed. This paper traces the Roman law way of thinking about blackmail into the Middle Ages and beyond, showing that it persisted even in the English common law.