Although it is well known that the criminal law's administration in nineteenth-century England altered decisively, little important change has been noted in the substantive criminal law. Yet change there was, but produced less through legislation (as was much administrative change) or even appeals court rulings than through everyday criminal justice practice. In particular, the effective meanings of legal terms central to the prosecution of homicide—terms such as provocation, intention, and insanity—were in motion during the nineteenth century as part of a broader redefining and reimagining of liability and responsibility. To grasp these often subtle shifts of meaning, we must look to the sites in which they occurred, the most important of which were the courtrooms of the assize courts, where the most serious offenses were tried.