The name given to the common priesthood of the Celtic peoples, although Druids were much more than simple priests - they were also teachers, seers, poets, judges, doctors, diviners and magicians. Their name is thought to come from drus, the ancient name for an oak tree, which was sacred to them.

The Druids were the unifying force between different Celtic tribes, their efforts preserving common culture, religion, history, laws, scholarship and science. Because Druids represented the most powerful force within the Celtic society and because their office was sacred, they had freedom to move as they wanted, being instrumental in stopping battles and forcing opposing factions to settle their disputes by arbitration. The Druids managed the highger legal systems and established colleges where pupils would receive up to twenty years' oral instruction before being admitted to their order. They were also responsible for the education of minstrels and bards, who received a similarly lengthy tuition.

Knowledge of the ancient Druids comes directly from contemporary writers, although little can be gleaned from these sources of their rituals and customs, which remain shrouded in mystery. Julius Caesar wrote that they had an intimate knowledge of the stars and their motions and of the universe and its size. They knew about the powers and authority of the gods, and taught the doctrine of the immortality of the soul. It appears that they also accepted the principles of reincarnation, for it was recorded that they allowed debts incurred in this lifetime to be repaid in the next.

Caesar significantly remarks that the first Druids came from Britain, and it is quite possible that this is a factual comment, for Druidism retains the appearance of an insular religion. It seems increasingly likely that the first Celts to arrive in Britain found there a religion controlled by priesthood. They simply adopted that religion and its priests, a supposition possibly supported by the religious importance placed by the Druids on ancient megalithic monuments such as Stonehenge. Yet there is a fundamental difference between the Celtic Druids and the megalithic priesthood. The Druids abandoned the great stones and reverted to natural shrines, a change that implies religious reformation, a reformation that may have come about with the first influx of Celtic peoples and a realisation by the priesthood that, if it were to retain its position of importance, it had to adapt its religion to match the beliefs of the new population.

It is almost impossible to put a date to this reformation, but it seems likely that the megalithic priests were in existence prior to 2000 BC. They appear to have enjoyed almost 1400 years of independence before the accepted date for the arrival of the first Celtic people in Britain, 600 BC. They then enjoyed a further 700 or 800 years of this independence before the first Christians arrived in Britain, and it was not until several centuries later that the pagan rituals of the Druids were finally eradicated from the Celtic Church when that was taken over by the Church of Rome.

The regularisation of the Celtic pantheon by the Druids was brought to an abrupt end by the Roman invasion, during which the priesthood and holy places were destroyed with systematic savagery. It was obvious to the Romans that the Druids represented the driving force behind Celtic power, and they thought that the only answer to such power was the sword. The nemeton, or holy groves, were put to the axe, and the Druids and their families slaughtered. Evidence of this massacre is to be found in the writings of Tacitus, although he refers to the slaughter of innocent women and children as 'heroism'. Having annihilated the religious order of the Celts, the Romans turned to the pen and assigned foreign and inappropriate names to the Celtic gods. This single act destroyed the Celtic way of life, but, more importantly for the student of Celtid times, made it impossible to determine the characteristics of many of the Celtic deities and their position within any pantheon that the Druids had devised.

If the Druids are indeed a priesthood that underwent a reformation with the arrival of the Celts in Britain, they must surely represent one of the oldest religions to have survived to the present, for there is today a great resurgence of the Druidic culture worldwide.