In Celtic Polytheism, the word Druid denotes the priestly class in ancient Celtic societies, which existed through much of Western Europe north of the Alps and in the British Isles. Druidic practices were part of the culture of all the tribal peoples called "Keltoi" and "Galatai" by Greeks and "Celtae" and "Galli" by Romans, which evolved into modern English "Celtic" and "Gaulish".

The word Druid (reconstructed as *druwis or *druwids in Old Celtic) is probably derived from Indo-European roots meaning "oak/strong" and "knowledge," although some believe it may be pre-Indo-European. By Ancient Greek writers, the earliest to discuss the Celts, the word is spelled Δρυίδης (Druides), and was associated with δρυς (drus "oak tree"). It appears in Old Irish as druí, giving draoi (magician) in Modern Irish and druidh (enchanter) and draoidh (magician) in Scottish Gaelic. The Old Irish druídecht gives Modern Irish draoiocht (magic). Welsh dryw (seer) may be cognate.

From what little we know of late Druidic practice, it appears deeply traditional, and conservative in the sense that Druids were conserving repositories of culture and lore. It is impossible now to judge whether this continuity had deep historical roots and originated in the social transformations of late La Tene time, or whether there had been a discontinuity and a Druidic religious innovation.

Our historical knowledge of Druids is very limited. Druidic lore consisted of a large number of verses learned by heart, and we are told that sometimes twenty years were required to complete the course of study. There may have been a Druidic teaching center on Ynys Môn (Anglesey)  centred on magical lakes, but what was taught, whether poetry, astronomy, or whether possibly even the Greek language, is conjecture. Of their oral literature of sacred songs, formulas for prayers and incantations, rules of divination and magic, not one verse has survived, even in translation, nor is there even a legend that we can call purely Druidic, without a Christian overlay or interpretation.

We find in Caesar's Gallic Wars and Civil Wars the first and fullest account of the Druids. Caesar notes that all men of any rank and dignity in Gaul were included either among the Druids or among the nobles, two separate classes. The Druids constituted the learned priestly class, and they were guardians of the unwritten ancient customary law and had the power of executing judgment, of which excommunication from society was the most dreaded. Druids were not a hereditary caste, though they enjoyed exemption from service in the field as well as from payment of taxes. The course of training to which a novice had to submit was protracted. All instruction was communicated orally, but for ordinary purposes, Caesar reports, the Gauls had a written language in which they used the Greek characters. No druidic documents have survived. "The principal point of their doctrine", says Caesar, "is that the soul does not die and that after death it passes from one body into another" (see metempsychosis). This led several ancient writers to the unlikely conclusion that the druids must have been influenced by the teachings of the Greek philosopher Pythagoras. Caesar also notes the druidic sense of the guardian spirit of the tribe, whom he translated as Dispater, with a general sense of Father Hades.

Writers like Diodorus and Strabo with less firsthand experience than Caesar, were of the opinion that this class included Druids, Bards and Vates (soothsayers).

Pomponius Mela is the first author who says that their instruction was secret and carried on in caves and forests. We know that certain groves within forests were sacred because Romans and Christians alike cut them down and burned the wood. Human sacrifice has sometimes been attributed to Druidism. While this may be Roman propaganda, human sacrifice was an old European inheritance. The Gauls were accustomed to offer human sacrifices, usually criminals.

Britain was a headquarters of Druidism, but once every year a general assembly of the order was held within the territories of the Carnutes in Gaul.

Cicero remarks on the existence among the Gauls of augurs or soothsayers, known by the name of Druids; he had made the acquaintance of one Divitiacus, an Aeduan. Diodorus informs us that a sacrifice acceptable to the gods must be attended by a Druid, for they are the intermediaries. Before a battle they often throw themselves between two armies to bring about peace.

Druids were seen as essentially non-Roman: a prescript of Augustus forbade Roman citizens to practise Druidical rites. In Strabo we find the Druids still acting as arbiters in public and private matters, but they no longer deal with cases of murder. Under Tiberius the Druids were suppressed by a decree of the Senate, but this had to be renewed by Claudius in 54 CE. In Pliny their activity is limited to the practice of medicine and sorcery. According to him, the Druids held the mistletoe in the highest veneration. Groves of oak were their chosen retreat. When thus found, the mistletoe was cut with a golden knife by a priest clad in a white robe, two white bulls being sacrificed on the spot.

Tacitus, in describing the attack made on the island of Mona (Anglesey - Ynys Mon in Welsh) by the Romans under Suetonius Paulinus, represents the legionaries as being awestruck on landing by the appearance of a band of Druids, who, with hands uplifted towards heaven, poured forth terrible imprecations on the heads of the invaders. The courage of the Romans, however, soon overcame such fears; the Britons were put to flight; and the sacred groves of Mona were cut down.

After the 1st century CE, the continental Druids disappeared entirely, and were only referred to on very rare occasions. Ausonius, for instance, apostrophizes the rhetorician Attius Patera as sprung from a race of Druids.

The story of Vortigern as reported by Nennius is one of the very few glimpses of Druidic survival in Britain after the Roman conquest. After being excommunicated by Germanus, the British leader invites twelve Druids to assist him. In Irish literature, however, the Druids are frequently mentioned, and their functions in the island seem to correspond fairly well to those of Gaul (the Modern Irish word for "magic", draíocht, derives from Old Irish druídecht). The functions of Druids we here find distributed amongst Druids, bards and poets, but even in very early times the poet has usurped many of the duties of the Druid (at least to judge from poetry) and finally supplants him with the spread of Christianity.

The Druids' influence was as much social as religious. They not only performed as a modern priests would, but were often the philosophers, scientists, lore-masters, teachers, judges and counsellors to the kings. The Druids linked the Celtic peoples with their numerous gods, the lunar calendar and the sacred natural order. They were suppressed in Gaul and Britain after the Roman conquests, but retained their influence in Ireland until the coming of Christianity. The Druids' roles were then assumed by the bishop and the abbot, who were usually not the same individual, however, and might find themselves in direct competition.

Nevertheless, much traditional rural religious practice can still be discerned from Christian interpretations and survives in practices like Halloween observances, corn dollies and other harvest rituals, the myths of Puck, woodwoses, "lucky" and "unlucky" plants and animals and the like. Orally-transmitted material may have exaggerated deep origins in antiquity, however, and is constantly subject to influence from surrounding culture.

The people of Flanders and the Low Countries remained pagan as late as the 7th century CE, when Saint Eligius travelled from Antwerp to Frisia, preaching and converting them to Christianity. One of the best glimpses of late Druidic practices comes from the Vita of Eligius written by saint Ouen, his contemporary and companion. Ouen drew together the familiar admonitions of Eligius to the pagans in Flanders. "It does not represent anything he said in a particular day in order" Ouen cautioned, "but is a digest of the precepts which he taught the people at all times."

Eligius in his sermons denounced "sacrilegious pagan customs." The following excerpted quotes from Ouen's Vita of Eligius are instructive, for the negative description they offer of some late druidic practices in Flanders:

"For no cause or infirmity should you consult magicians, diviners, sorcerers or incantators, or presume to question them."

"Do not observe auguries or violent sneezing or pay attention to any little birds singing along the road. If you are distracted on the road or at any other work, make the sign of the cross and say your Sunday prayers with faith and devotion and nothing inimical can hurt you."

"No Christian should be concerned about which day he leaves home or which day he returns, because God has made all days. No influence attaches to the first work of the day or the [phase of the] moon; nothing is ominous or ridiculous about the Calends of January [what we would call New Year's Day ].

"[Do not] make vetulas,*, little deer or iotticos or set tables [for the house-elf, compare Puck] at night or exchange New Years' gifts or supply superfluous drinks [a Yule custom]."

• Vetula, a little figure of the Old Woman. A Roman would have equated her with Hecate, but precisely who the Old Woman was and what she meant in the pagan Low Countries cannot be determined.

"No Christian gives credence to impurity or sits in incantation, because the work is diabolic. No Christian on the feast of Saint John* or the solemnity of any other saint performs solestitia [summer solstice rites] or dancing or leaping or diabolical chants."

• The Christian summer solstice feast of Saint John the Baptist is still celebrated with bonfires on June 24th, though the precession of the equinoxes has shifted the actual equinox forward in the calendar. See entry Midsummer.

"No Christian should presume to invoke the name of a demon, not Neptune or Orcus* or Diana or Minerva or Geniscus or believe in these inept beings in any way. No one should observe Jove's day in idleness without holy festivities not in May or any other time, not days of larvae** or mice or any day but Sunday. No Christian should make or render any devotion to the gods of the trivium, where three roads meet [cf. Hecate], to the fanes or the rocks, or springs or groves or corners."

• Orcus, a chthonic Etruscan/Roman god of the underworld, who enforced the sacredness of oaths and avenged the broken word. (An essay on Hades/Orcus.)

• Larvae ("malignant spirits") in this Latin text more specifically refer to the Roman Feast of the Lemures, propitiating the dead, rather than to the Celtic propitiation, which was at Samhain.

"None should presume to hang any phylacteries* from the neck of man nor beast, even if they are made by priests and it is said that they contain holy things and divine scripture, because there is no remedy of Christ in these things but only the devil's poison."

• Gregory of Tours set great store by phylacteries.

"None should presume to make lustrations or incantations with herbs, or to pass cattle through a hollow tree or ditch because this is to consecrate them to the devil. No woman should presume to hang amber from her neck or call upon Minerva or other ill-starred beings in their weaving or dyeing but in all works give thanks only to Christ and confide in the power of his name with all your hearts. None should presume to shout when the moon is obscured, for by God's order eclipses happen at certain times. Nor should they fear the new moon or abandon work because of it. For God made the moon for this, to mark time and temper the darkness of night, not impede work nor make men mad as the foolish imagine, who believe lunatics are invaded by demons from the moon. None should call the sun or moon lord or swear by them because they are God's creatures and they serve the needs of men by God's order."

"No one should tell fate or fortune or horoscopes by them as those do who believe that a person must be what he was born to be."

"Above all, should any infirmity occur, do not seek incantators or diviners or sorcerers or magicians, do not use diabolic phylacteries through springs and groves or crossroads. But let the invalid confide solely in the mercy of God and take the body and blood of Christ with faith and devotion and ask the church faithfully for blessing and oil, with which he might anoint his body in the name of Christ and, according to the apostle, "the prayer of faith will save the infirm and the Lord will relieve him."

"Diabolical games and dancing or chants of the gentiles will be forbidden. No Christian will do them because he thus makes himself pagan. Nor is it right that diabolical canticles should proceed from a Christian mouth where the sacrament of Christ is placed, which it becomes always to praise God. Therefore, brothers, spurn all inventions of the enemy with all your heart and flee these sacrileges with all horror. Venerate no creature beyond God and his saints. Shun springs and arbors which they call sacred. You are forbidden to make the crook which they place on the crossroads and wherever you find one you should burn it with fire. For you must believe that you can be saved by no other art than the invocation and cross of Christ. For how will it be if groves where these miserable men make their devotions, are felled and the wood from them given to the furnace? See how foolish man is, to offer honor to insensible, dead trees and despise the precepts of God almighty. Do not believe that the sky or the stars or the earth or any creature should be adored beyond God for he created and disposes of them all."

In the 18th century, England and Wales experienced a Druid revival, inspired by, among others, John Aubrey, John Toland and William Stukeley. There is strong evidence to suggest that William Blake was involved in the Druid revival and may have been an Archdruid. Aubrey was the first modern writer to connect Stonehenge and other megalithic monuments with Druidry, a misconception that shaped ideas of Druidry during much of the 19th century. Modern Druidic groups have their roots in this revival, and some claim that Aubrey was an archdruid in possession of an uninterrupted tradition of Druidic knowledge, though Aubrey, an uninhibited collector of lore and gossip, never entered a corroborating word in his voluminous surviving notebooks. Toland was fascinated by Aubrey's Stonehenge theories, and wrote his own book, without crediting Aubrey. He has also been claimed as an Archdruid. The Ancient Druid Order claim that Toland held a gathering of Druids from all over Britain and Ireland in a London tavern, the Appletree, in 1717. The Ancient Order of Druids itself was founded in 1781, led by Henry Hurle and apparently incorporating Masonic ideas. A central figure of the Druidic revival is Edward Williams, better known as Iolo Morganwg. His writings, published posthumously as The Iolo Manuscripts (1848), and Barddas (1862), remain influential in the contemporary Druidic movements. Williams claimed to have collected ancient knowledge in a "Gorsedd of Bards of the Isles of Britain" he had organized, but in the 1970s, draft manuscripts of the texts were discovered among Williams' papers, exposing the texts as his own compositions.

An unfortunate result of this druid revival, which took place before the advent of modern archaeological and historical methods, is that it has shaped public perceptions of historical Druidry and continues to shape some modern forms of it. The British Museum website is quite blunt:

"Modern Druids have no direct connection to the Druids of the Iron Age. Many of our popular ideas about the Druids are based on the misunderstandings and misconceptions of scholars 200 years ago. These ideas have been superceded by later study and discoveries"

Some strands of modern Druidism (a.k.a. Modern Druidry), such as the Order of Bards, Ovates and Druids (OBOD), are a continuation of the 18th-century revival and thus are built largely around writings produced in the 18th century and later. Members may be Neo-Pagan, Christian, or non-specifically monotheistic.

Other strands could be classified as eclectic NeoPaganism, and may mix elements from other cultures such as Shamanism and Native American; they are typically more interested in modern experience than in scholarship.

A third strand, more akin to Celtic Reconstructionism, rejects the 18th-century revival entirely and tracks the latest scholarship on the relatively sparse Roman and early medieval written sources, archaeology, and comparative linguistics in an attempt to get as close as possible to Ancient Druidry.

Modern Druidism has two strands, the cultural and the religious. Cultural Druids hold a competition of poetry, literature and music known as the Eisteddfod amongst the Celtic peoples (Welsh, Irish, Cornish, Breton, etc).

It is not always easy to distinguish between the two strands, because religiously-oriented Druid orders may welcome members of any or no religious background while culturally-oriented orders may not inquire about the religious beliefs of members. Both types of Druid order, then, may contain both religiously-oriented and non-religiously oriented members. Many notable Britons have been initiated into Druidic orders, including Winston Churchill. Churchill's case illustrates the difficulty of distinguishing between the two strands, because historians are not even certain which order he joined, the Ancient Order of Druids or the Ancient and Archaeological Order of Druids, let alone for what purpose he joined.

Fragments of a Druidic Lunar Calendar may be preserved in the Coligny calendar, fragments of a calendar engraved on a bronze tablet, discovered in 1897.