Public Domain"Arch-Druid in his full Judicial Costume" etching from Charles Knight, Old England: A Pictorial Museum (1845)

Druids were the priests, political advisors, teachers, healers and arbitrators of Celtic Britain. They were trained in "universities" and their knowledge was learned by rote. This knowledge was said to have been secretly taught in caves and forests. To become a druid was alleged to  take up to twenty years of study. Druids were entitled to speak before the king in council, acted as ambassadors and judges, and composed verse. They practiced animal and human sacrifice, and worshipped in oak groves and near water. The Isle of Anglesey was a centre for them.

Little is known for certain about the druids. Most of our evidence comes from hostile Roman sources. They were credited with magical powers and sometimes referred to as magi. The Greek scholar Alexander Cornelius Polyhistor called them "philosophers" and said that they believed in reincarnation. The "wicker man," in which men were said to have been sacrificially burned alive, is nemtioned in Caesar's Gallic War. It has been said that the druids could never have been associated with such stone circles as Stonehenge; because the circles were built in the Neolithic and Bronze Ages, long before their time: but this is equivalent to saying that the archbishop Rowan Williams could never have been in Canterbury Cathedral because it was built in the Middle Ages, long before his time!

The druidic practice of human sacrifice horrified the Romans. The druids of Anglesey were exterminated in 61 A.D., three years before the great Fire of Rome. Druidism then ceased to be a religious force in Britain, though it may have survived in Ireland as late as the 7th century. An attempt to revive it was made in the 18th century in the form of the Ancient Order of Druids, which meets annually at Stonehenge on Midsummer's Day.