The Bacchanalia were unofficial, privately funded popular Roman festivals of Bacchus, based on various ecstatic elements of the Greek Dionysia. They were almost certainly associated with Rome's native cult of Liber, and probably arrived in Rome itself around 200 BC. Like all mystery religions of the ancient world, very little is known of their rites. They seem to have been popular and well-organised throughout the central and southern Italian peninsula.

Livy, writing some 200 years after the event, offers a scandalized and extremely colourful account of the Bacchanalia, with frenzied rites, sexually violent initiations of both sexes, all ages and all social classes; he represents the cult as a murderous instrument of conspiracy against the state. Livy claims that seven thousand cult leaders and followers were arrested, and that most were executed. Livy believed the Bacchanalia scandal to be one of several indications of Rome's inexorable moral decay. Modern scholars take a skeptical approach to Livy's allegations.

The cult was not banned. Senatorial legislation to reform the Bacchanalia in 186 BC attempted to control their size, organisation, and priesthoods, under threat of the death penalty. This may have been motivated less by the kind of lurid and dramatic rumours that Livy describes than by the Senate's determination to assert its civil, moral and religious authority over Rome and its allies, after the prolonged social, political and military crisis of the Second Punic War (218–201 BC). The reformed Bacchanalia rites may have been merged with the Liberalia festival. Bacchus, Liber and Dionysus became virtually interchangeable from the late Republican era (133 BC and onward), and their mystery cults persisted well into the Principate of Roman Imperial era.