There are several ancient and Byzantine accounts of the assassination of the emperor Gallienus (253-68), who was murdered while besieging a rebellious general, Aureolus, at Milan in early September 268. But few of these accounts agree fully on those who were responsible for the killing, and some are openly contradictory. Modern scholars, in turn, have not been in accord on the members of the conspiracy. A close examination of these sources demonstrates that only three men are reliably attested to have been involved: Heraclianus, Gallienus’ praetorian prefect; Claudius II Gothicus, emperor in 268-70; and Aurelian, emperor in 270-75. In addition, the investigation provides further insights into the development of the historical traditions pertaining to this period of Roman history.