The concept that the Hallstatt and La Tène cultures could be seen not just as chronological periods but as "Culture Groups", entities composed of people of the same ethnicity and language, had started to grow by the end of the 19th century. At the beginning of the 20th century the belief that these "Culture Groups" could be thought of in racial or ethnic terms was held by Gordon Childe, whose theory was influenced by the writings of Gustaf Kossinna. As the 20th century progressed, the ethnic interpretation of La Tène culture became more strongly rooted, and any findings of La Tène culture and flat inhumation cemeteries were linked to the Celts and the Celtic language.

In various academic disciplines the Celts were considered a Central European Iron Age phenomenon, through the cultures of Hallstatt and La Tène. However, archaeological finds from the Halstatt and La Tène culture were rare in Iberia, southwestern France, northern and western Britain, southern Ireland and Galatia and did not provide enough evidence for a culture like that of Central Europe. It is equally difficult to maintain that the origin of the Iberian Celts can be linked to the preceding Urnfield culture. This has resulted in a newer theory that introduces a 'proto-Celtic' substratum and a process of Celticisation, having its initial roots in the Bronze Age Bell Beaker culture.

The La Tène culture developed and flourished during the late Iron Age (from 450 BC to the Roman conquest in the 1st century BC) in eastern France, Switzerland, Austria, southwest Germany, the Czech Republic, Slovakia and Hungary. It developed out of the Hallstatt culture without any definite cultural break, under the impetus of considerable Mediterranean influence from Greek, and later Etruscan civilisations. A shift of settlement centres took place in the 4th century. The western La Tène culture corresponds to historical Celtic Gaul. Whether this means that the whole of La Tène culture can be attributed to a unified Celtic people is difficult to assess; archaeologists have repeatedly concluded that language and material culture do not necessarily run parallel. Frey notes that in the 5th century, "burial customs in the Celtic world were not uniform; rather, localised groups had their own beliefs, which, in consequence, also gave rise to distinct artistic expressions". Thus, while the La Tène culture is certainly associated with the Gauls, the presence of La Tène artefacts may be due to cultural contact and does not imply the permanent presence of Celtic speakers.

The Greek historian Ephorus of Cyme in Asia Minor, writing in the 4th century BC, believed the Celts came from the islands off the mouth of the Rhine and were "driven from their homes by the frequency of wars and the violent rising of the sea". Polybius published a history of Rome about 150 BC in which he describes the Gauls of Italy and their conflict with Rome. Pausanias in the 2nd century AD says that the Gauls "originally called Celts", "live on the remotest region of Europe on the coast of an enormous tidal sea". Posidonius described the southern Gauls about 100 BC. Though his original work is lost, later writers such as Strabo used it. The latter, writing in the early 1st century AD, deals with Britain and Gaul as well as Hispania, Italy, and Galatia. Caesar wrote extensively about his Gallic Wars in 58–51 BC. Diodorus Siculus wrote about the Celts of Gaul and Britain in his 1st-century history.

Diodorus Siculus and Strabo both suggest that the heartland of the people they call Celts was in southern Gaul. The former says that the Gauls were to the north of the Celts, but that the Romans referred to both as Gauls (linguistically the Gauls were certainly Celts). Before the discoveries at Hallstatt and La Tène, it was generally considered that the Celtic heartland was southern Gaul, see Encyclopædia Britannica for 1813.

The Romans knew the Celts then living in present-day France as Gauls. The territory of these peoples probably included the Low Countries, the Alps and present-day northern Italy. Julius Caesar in his Gallic Wars described the 1st-century BC descendants of those Gauls.

Eastern Gaul became the centre of the western La Tène culture. In later Iron Age Gaul, the social organisation resembled that of the Romans, with large towns. From the 3rd century BC the Gauls adopted coinage. Texts with Greek characters from southern Gaul have survived from the 2nd century BC.

Greek traders founded Massalia about 600 BC, with some objects (mostly drinking ceramic vessels) being traded up the Rhône valley. But trade became disrupted soon after 500 BC and re-oriented over the Alps to the Po valley in the Italian peninsula. The Romans arrived in the Rhone valley in the 2nd century BC and encountered a mostly Celtic-speaking Gaul. Rome wanted land communications with its Iberian provinces and fought a major battle with the Saluvii at Entremont in 124–123 BC. Gradually Roman control extended, and the Roman province of Gallia Transalpina developed along the Mediterranean coast. The Romans knew the remainder of Gaul as Gallia Comata, 'Long-haired Gaul'.

In 58 BC, the Helvetii planned to migrate westward but Julius Caesar forced them back. He then became involved in fighting the various tribes in Gaul, and by 55 BC had overrun most of Gaul. In 52 BC, Vercingetorix led a revolt against Roman occupation but was defeated at the Battle of Alesia and surrendered.

Following the Gallic Wars of 58–51 BC, Caesar's Celtica formed the main part of Roman Gaul, becoming the province of Gallia Lugdunensis. This territory of the Celtic tribes was bounded on the south by the Garonne and on the north by the Seine and the Marne. The Romans attached large swathes of this region to neighbouring provinces Belgica and Aquitania, particularly under Augustus.

Place- and personal-name analysis and inscriptions suggest that Gaulish was spoken over most of what is now France.

Until the end of the 19th century, traditional scholarship dealing with the Celts did acknowledge their presence in the Iberian Peninsula as a material culture relatable to the Hallstatt and La Tène cultures. However, since according to the definition of the Iron Age in the 19th century Celtic populations were supposedly rare in Iberia and did not provide a cultural scenario that could easily be linked to that of Central Europe, the presence of Celtic culture in that region was generally not fully recognised. Modern scholarship, however, has proven that Celtic presence and influences were most substantial in what is today Spain and Portugal (with perhaps the highest settlement saturation in Western Europe), particularly in the central, western and northern regions.

In addition to Gauls infiltrating from the north of the Pyrenees, the Roman and Greek sources mention Celtic populations in three parts of the Iberian Peninsula: the eastern part of the Meseta (inhabited by the Celtiberians), the southwest (Celtici, in modern-day Alentejo) and the northwest (Gallaecia and Asturias). A modern scholarly review found several archaeological groups of Celts in Spain:

• The Celtiberian group in the Upper-Douro Upper-Tagus Upper-Jalón area. Archaeological data suggest a continuity at least from the 6th century BC. In this early period, the Celtiberians inhabited in hill-forts (Castros). Around the end of the 3rd century BC, Celtiberians adopted more urban ways of life. From the 2nd century BC, they minted coins and wrote inscriptions using the Celtiberian script. These inscriptions make the Celtiberian Language the only Hispano-Celtic language classified as Celtic with unanimous agreement. In the late period, before the Roman Conquest, both archaeological evidence and Roman sources suggest that the Celtiberians were expanding into different areas in the Peninsula (e.g. Celtic Baeturia).
• The Vetton group in the western Meseta, between the Tormes, Douro and Tagus Rivers. They were characterised by the production of Verracos, sculptures of bulls and pigs carved in granite.
• The Vaccean group in the central Douro valley. They were mentioned by Roman sources already in the 220 BC. Some of their funerary rituals suggest strong influences from their Celtiberian neighbours.

• The Castro Culture in northwestern Iberia, modern day Galicia and Northern Portugal. Its high degree of continuity, from the Late Bronze Age, makes it difficult to support that the introduction of Celtic elements was due to the same process of Celticisation of the western Iberia, from the nucleus area of Celtiberia. Two typical elements are the sauna baths with monumental entrances, and the "Gallaecian Warriors", stone sculptures built in the 1st century AD. A large group of Latin inscriptions contain Celtic linguistic features, while others are similar to those found in the non-Celtic Lusitanian language.
• The Astures and the Cantabri. This area was romanised late, as it was not conquered by Rome until the Cantabrian Wars of 29–19 BC.
• Celts in the southwest, in the area Strabo called Celtica

The origins of the Celtiberians might provide a key to understanding the Celticisation process in the rest of the Peninsula. The process of Celticisation of the southwestern area of the peninsula by the Keltoi and of the northwestern area is, however, not a simple Celtiberian question. Recent investigations about the Callaici and Bracari in northwestern Portugal are providing new approaches to understanding Celtic culture (language, art and religion) in western Iberia.

John T. Koch of Aberystwyth University suggested that Tartessian inscriptions of the 8th century BC might be classified as Celtic. This would mean that Tartessian is the earliest attested trace of Celtic by a margin of more than a century.

In Germany by the late Bronze Age, the Urnfield culture (c. 1300 BC – c. 750 BC) had replaced the Bell Beaker, Unetice and Tumulus cultures in central Europe, whilst the Nordic Bronze Age had developed in Scandinavia and northern Germany. The Hallstatt culture, which had developed from the Urnfield culture, was the predominant Western and Central European culture from the 12th to 8th centuries BC and during the early Iron Age (8th to 6th centuries BC). It was followed by the La Tène culture (5th to 1st centuries BC).

The people who had adopted these cultural characteristics in central and southern Germany are regarded as Celts. Celtic cultural centres developed in central Europe during the late Bronze Age (c. 1200 BC until 700 BC). Some, like the Heuneburg, the oldest city north of the Alps, grew to become important cultural centres of the Iron Age in Central Europe, that maintained trade routes to the Mediterranean. In the 5th century BC the Greek historian Herodotus mentioned a Celtic city at the Danube – Pyrene, that historians attribute to the Heuneburg. Beginning around 700 BC (or later), Germanic peoples (Germanic tribes) from southern Scandinavia and northern Germany expanded south and gradually replaced the Celtic peoples in Central Europe.

The Canegrate culture represented the first migratory wave of the proto-Celtic population from the northwest part of the Alps that, through the Alpine passes, had already penetrated and settled in the western Po valley between Lake Maggiore and Lake Como (Scamozzina culture). It has also been proposed that a more ancient proto-Celtic presence can be traced back to the beginning of the Middle Bronze Age, when North Westwern Italy appears closely linked regarding the production of bronze artefacts, including ornaments, to the western groups of the Tumulus culture. La Tène cultural material appeared over a large area of mainland Italy, the southernmost example being the Celtic helmet from Canosa di Puglia.

Italy is home to Lepontic, the oldest attested Celtic language (from the 6th century BC). Anciently spoken in Switzerland and in Northern-Central Italy, from the Alps to Umbria. According to the Recueil des Inscriptions Gauloises, more than 760 Gaulish inscriptions have been found throughout present-day France – with the notable exception of Aquitaine – and in Italy, which testifies the importance of Celtic heritage in the peninsula.

In 391 BC, Celts "who had their homes beyond the Alps streamed through the passes in great strength and seized the territory that lay between the Apennine Mountains and the Alps" according to Diodorus Siculus. The Po Valley and the rest of northern Italy (known to the Romans as Cisalpine Gaul) was inhabited by Celtic-speakers who founded cities such as Milan. Later the Roman army was routed at the battle of Allia and Rome was sacked in 390 BC by the Senones.

At the battle of Telamon in 225 BC, a large Celtic army was trapped between two Roman forces and crushed.

The defeat of the combined Samnite, Celtic and Etruscan alliance by the Romans in the Third Samnite War sounded the beginning of the end of the Celtic domination in mainland Europe, but it was not until 192 BC that the Roman armies conquered the last remaining independent Celtic kingdoms in Italy.

The Celts also expanded down the Danube river and its tributaries. One of the most influential tribes, the Scordisci, established their capital at Singidunum (present-day Belgrade, Serbia) in the 3rd century BC. The concentration of hill-forts and cemeteries shows a dense population in the Tisza valley of modern-day Vojvodina, Serbia, Hungary and into Ukraine. Expansion into Romania was however blocked by the Dacians.

The Serdi were a Celtic tribe inhabiting Thrace. They were located around and founded Serdika (Bulgarian: Сердика, Latin: Ulpia Serdica, Greek: Σαρδῶν πόλις), now Sofia in Bulgaria, which reflects their ethnonym. They would have established themselves in this area during the Celtic migrations at the end of the 4th century BC, though there is no evidence for their existence before the 1st century BC. Serdi are among traditional tribal names reported into the Roman era. They were gradually Thracianized over the centuries but retained their Celtic character in material culture up to a late date. According to other sources they may have been simply of Thracian origin, according to others they may have become of mixed Thraco-Celtic origin. Further south, Celts settled in Thrace (Bulgaria), which they ruled for over a century, and Anatolia, where they settled as the Galatians (see also: Gallic Invasion of Greece). Despite their geographical isolation from the rest of the Celtic world, the Galatians maintained their Celtic language for at least 700 years. St Jerome, who visited Ancyra (modern-day Ankara) in 373 AD, likened their language to that of the Treveri of northern Gaul.

For Venceslas Kruta, Galatia in central Turkey was an area of dense Celtic settlement.

The Boii tribe gave their name to Bohemia, Bologna and possibly Bavaria, and Celtic artefacts and cemeteries have been discovered further east in what is now Poland and Slovakia. A Celtic coin (Biatec) from Bratislava's mint was displayed on the old Slovak 5-crown coin.

As there is no archaeological evidence for large-scale invasions in some of the other areas, one current school of thought holds that Celtic language and culture spread to those areas by contact rather than invasion. However, the Celtic invasions of Italy and the expedition in Greece and western Anatolia, are well documented in Greek and Latin history.

There are records of Celtic mercenaries in Egypt serving the Ptolemies. Thousands were employed in 283–246 BC and they were also in service around 186 BC. They attempted to overthrow Ptolemy II.

All living Celtic languages today belong to the Insular Celtic languages, derived from the Celtic languages spoken in Iron Age Britain and Ireland. They separated into a Goidelic and a Brittonic branch early on. By the time of the Roman conquest of Britain in the 1st century AD, the Insular Celts were made up of the Celtic Britons, the Gaels (or Scoti), and the Picts (or Caledonians). The renown of insular Celts has caused a popular belief that Celtic clans only lived in the British Isles.

Linguists have debated whether a Celtic language came to the British Isles and then split, or whether the two branches arrived separately. The older view was that Celtic influence in the Isles was the result of successive migrations or invasions from the European mainland by diverse Celtic-speaking peoples over several centuries, accounting for the P-Celtic vs. Q-Celtic isogloss. This view has been challenged by the hypothesis that the islands' Celtic languages form an Insular Celtic dialect group. In the 19th and 20th centuries, scholars often dated the "arrival" of Celtic culture in Britain (via an invasion model) to the 6th century BC, corresponding to archaeological evidence of Hallstatt influence and the appearance of chariot burials in what is now England. Cunliffe and Koch propose in their newer 'Celtic from the West' theory that Celtic languages reached the Isles earlier, with the Bell Beaker culture c.2500 BC, or even before this. More recently, a major archaeogenetics study uncovered a migration into southern Britain in the Bronze Age from 1300 to 800 BC. The newcomers were genetically most similar to ancient individuals from Gaul. From 1000 BC, their genetic marker swiftly spread through southern Britain, but not northern Britain. The authors see this as a "plausible vector for the spread of early Celtic languages into Britain". There was much less immigration during the Iron Age, so it is likely that Celtic reached Britain before then. Cunliffe suggests that a branch of Celtic was already spoken in Britain, and the Bronze Age migration introduced the Brittonic branch.

Like many Celtic peoples on the mainland, the Insular Celts followed an Ancient Celtic religion overseen by druids. Some of the southern British tribes had strong links with Gaul and Belgica, and minted their own coins. During the Roman occupation of Britain, a Romano-British culture emerged in the southeast. The Britons and Picts in the north, and the Gaels of Ireland, remained outside the empire. During the end of Roman rule in Britain in the 400s AD, there was significant Anglo-Saxon settlement of eastern and southern Britain, and some Gaelic settlement of its western coast. During this time, some Britons migrated to the Armorican peninsula, where their culture became dominant. Meanwhile, much of northern Britain (Scotland) became Gaelic. By the 10th century AD, the Insular Celtic peoples had diversified into the Brittonic-speaking Welsh (in Wales), Cornish (in Cornwall), Bretons (in Brittany) and Cumbrians (in the Old North); and the Gaelic-speaking Irish (in Ireland), Scots (in Scotland) and Manx (on the Isle of Man).

Classical writers did not call the inhabitants of Britain and Ireland Celtae or Κελτοί (Keltoi), leading some scholars to question the use of the term 'Celt' for the Iron Age inhabitants of those islands. The first historical account of the islands was by the Greek geographer Pytheas, who sailed around what he called the "Pretannikai nesoi" (the "Pretannic isles") around 310–306 BC. In general, classical writers referred to the Britons as Pretannoi (in Greek) or Britanni (in Latin). Strabo, writing in Roman times, distinguished between the Celts and Britons. However, Roman historian Tacitus says the Britons resembled the Celts of Gaul in customs and religion.

Under Caesar the Romans conquered Celtic Gaul, and from Claudius onward the Roman empire absorbed parts of Britain. Roman local government of these regions closely mirrored pre-Roman tribal boundaries, and archaeological finds suggest native involvement in local government.

The native peoples under Roman rule became Romanised and keen to adopt Roman ways. Celtic art had already incorporated classical influences, and surviving Gallo-Roman pieces interpret classical subjects or keep faith with old traditions despite a Roman overlay.

The Roman occupation of Gaul, and to a lesser extent of Britain, led to Roman-Celtic syncretism. In the case of the continental Celts, this eventually resulted in a language shift to Vulgar Latin, while the Insular Celts retained their language.

There was also considerable cultural influence exerted by Gaul on Rome, particularly in military matters and horsemanship, as the Gauls often served in the Roman cavalry. The Romans adopted the Celtic cavalry sword, the spatha, and Epona, the Celtic horse goddess.

To the extent that sources are available, they depict a pre-Christian Iron Age Celtic social structure based formally on class and kingship, although this may only have been a particular late phase of organisation in Celtic societies. Patron-client relationships similar to those of Roman society are also described by Caesar and others in the Gaul of the 1st century BC.

In the main, the evidence is of tribes being led by kings, although some argue that there is also evidence of oligarchical republican forms of government eventually emerging in areas which had close contact with Rome. Most descriptions of Celtic societies portray them as being divided into three groups: a warrior aristocracy; an intellectual class including professions such as druid, poet, and jurist; and everyone else. In historical times, the offices of high and low kings in Ireland and Scotland were filled by election under the system of tanistry, which eventually came into conflict with the feudal principle of primogeniture in which succession goes to the first-born son.

Little is known of family structure among the Celts. Patterns of settlement varied from decentralised to urban. The popular stereotype of non-urbanised societies settled in hillforts and duns, drawn from Britain and Ireland (there are about 3,000 hill forts known in Britain) contrasts with the urban settlements present in the core Hallstatt and La Tène areas, with the many significant oppida of Gaul late in the first millennium BC, and with the towns of Gallia Cisalpina.

Slavery, as practised by the Celts, was very likely similar to the better documented practice in ancient Greece and Rome. Slaves were acquired from war, raids, and penal and debt servitude. Slavery was hereditary, though manumission was possible. The Old Irish and Welsh words for 'slave', cacht and caeth respectively, are cognate with Latin captus 'captive' suggesting that the slave trade was an early means of contact between Latin and Celtic societies. In the Middle Ages, slavery was especially prevalent in the Celtic countries. Manumissions were discouraged by law and the word for 'female slave', cumal, was used as a general unit of value in Ireland.

There are only very limited records from pre-Christian times written in Celtic languages. These are mostly inscriptions in the Roman and sometimes Greek alphabets. The ogham script, an Early Medieval alphabet, was mostly used in early Christian times in Ireland and Scotland (but also in Wales and England), and was only used for ceremonial purposes such as inscriptions on gravestones. The available evidence is of a strong oral tradition, such as that preserved by bards in Ireland, and eventually recorded by monasteries. Celtic art also produced a great deal of intricate and beautiful metalwork, examples of which have been preserved by their distinctive burial rites.

In some regards the Atlantic Celts were conservative: for example, they still used chariots in combat long after they had been reduced to ceremonial roles by the Greeks and Romans. However, despite being outdated, Celtic chariot tactics were able to repel the invasions of Britain attempted by Julius Caesar.

According to Diodorus Siculus:

The Gauls are tall of body with rippling muscles and white of skin and their hair is blond, and not only naturally so for they also make it their practice by artificial means to increase the distinguishing colour which nature has given it. For they are always washing their hair in limewater and they pull it back from the forehead to the nape of the neck, with the result that their appearance is like that of Satyrs and Pans since the treatment of their hair makes it so heavy and coarse that it differs in no respect from the mane of horses. Some of them shave the beard but others let it grow a little; and the nobles shave their cheeks but they let the moustache grow until it covers the mouth.

During the later Iron Age the Gauls generally wore long-sleeved shirts or tunics and long trousers (called braccae by the Romans). Clothes were made of wool or linen, with some silk being used by the rich. Cloaks were worn in the winter. Brooches and armlets were used, but the most famous item of jewellery was the torc, a neck collar of metal, sometimes gold. The horned Waterloo Helmet in the British Museum, which long set the standard for modern images of Celtic warriors, is in fact a unique survival, and may have been a piece for ceremonial rather than military wear.

Archaeological evidence suggests that the pre-Roman Celtic societies were linked to the network of overland trade routes that spanned Eurasia. Archaeologists have discovered large prehistoric trackways crossing bogs in Ireland and Germany. Due to their substantial nature, these are believed to have been created for wheeled transport as part of an extensive roadway system that facilitated trade. The territory held by the Celts contained tin, lead, iron, silver and gold. Celtic smiths and metalworkers created weapons and jewellery for international trade, particularly with the Romans.

The myth that the Celtic monetary system consisted of wholly barter is a common one, but is in part false. The monetary system was complex and is still not understood (much like the late Roman coinages), and due to the absence of large numbers of coin items, it is assumed that "proto-money" was used. This included bronze items made from the early La Tène period and onwards, which were often in the shape of axeheads, rings, or bells. Due to the large number of these present in some burials, it is thought they had a relatively high monetary value, and could be used for "day to day" purchases. Low-value coinages of potin, a bronze alloy with high tin content, were minted in most Celtic areas of the continent and in South-East Britain prior to the Roman conquest of these lands. Higher-value coinages, suitable for use in trade, were minted in gold, silver, and high-quality bronze. Gold coinage was much more common than silver coinage, despite being worth substantially more, as while there were around 100 mines in Southern Britain and Central France, silver was more rarely mined. This was due partly to the relative sparsity of mines and the amount of effort needed for extraction compared to the profit gained. As the Roman civilisation grew in importance and expanded its trade with the Celtic world, silver and bronze coinage became more common. This coincided with a major increase in gold production in Celtic areas to meet the Roman demand, due to the high value Romans put on the metal. The large number of gold mines in France is thought to be a major reason why Caesar invaded.

Very few reliable sources exist regarding Celtic views on gender roles, though some archaeological evidence suggests their views may have differed from those of the Greco-Roman world, which tended to be less egalitarian. Some Iron Age burials in northeastern Gaul suggest women may have had roles in warfare during the earlier La Tène period, but the evidence is far from conclusive. Celtic individuals buried with both female jewellery and weaponry have been found, such as the Vix Grave in northeastern Gaul, and there are questions about the gender of some individuals buried with weaponry. However, it has been suggested that the weapons indicate high social rank rather than masculinity.

Most written accounts of the Ancient Celts are from the Romans and Greeks, though it is not clear how accurate these are. Roman historians Ammianus Marcellinus and Tacitus mentioned Celtic women inciting, participating in, and leading battles. Plutarch reports that Celtic women acted as ambassadors to avoid a war among Celtic chiefdoms in the Po valley during the 4th century BC. Posidonius' anthropological comments on the Celts had common themes, primarily primitivism, extreme ferocity, cruel sacrificial practices, and the strength and courage of their women. Cassius Dio suggests there was great sexual freedom among women in Celtic Britain:

... a very witty remark is reported to have been made by the wife of Argentocoxus, a Caledonian, to Julia Augusta. When the empress was jesting with her, after the treaty, about the free intercourse of her sex with men in Britain, she replied: "We fulfill the demands of nature in a much better way than do you Roman women; for we consort openly with the best men, whereas you let yourselves be debauched in secret by the vilest". Such was the retort of the British woman.

Barry Cunliffe writes that such references are "likely to be ill-observed" and meant to portray the Celts as outlandish "barbarians". Historian Lisa Bitel argues the descriptions of Celtic women warriors are not credible. She says some Roman and Greek writers wanted to show that the barbarian Celts lived in "an upside-down world ... and a standard ingredient in such a world was the manly warrior woman".

The Greek philosopher Aristotle wrote in his Politics that the Celts of southeastern Europe approved of male homosexuality. Greek historian Diodorus Siculus wrote in his Bibliotheca historica that although Gaulish women were beautiful, the men had "little to do with them" and it was a custom for men to sleep on animal skins with two younger males. He further claimed that "the young men will offer themselves to strangers and are insulted if the offer is refused". His claim was later repeated by Greco-Roman writers Athenaeus and Ammianus. H. David Rankin, in Celts and the Classical World, suggests some of these claims refer to bonding rituals in warrior groups, which required abstinence from women at certain times, and says it probably reflects "the warlike character of early contacts between the Celts and the Greeks".

Under Brehon Law, which was written down in early Medieval Ireland after conversion to Christianity, a woman had the right to divorce her husband and gain his property if he was unable to perform his marital duties due to impotence, obesity, homosexual inclination or preference for other women.

Celtic art is generally used by art historians to refer to art of the La Tène period across Europe, while the Early Medieval art of Britain and Ireland, that is what "Celtic art" evokes for much of the general public, is called Insular art in art history. Both styles absorbed considerable influences from non-Celtic sources, but retained a preference for geometrical decoration over figurative subjects, which are often extremely stylised when they do appear; narrative scenes only appear under outside influence. Energetic circular forms, triskeles and spirals are characteristic. Much of the surviving material is in precious metal, which no doubt gives a very unrepresentative picture, but apart from Pictish stones and the Insular high crosses, large monumental sculpture, even with decorative carving, is very rare; possibly it was originally common in wood. Celts were also able to create developed musical instruments such as the carnyces, these famous war trumpets used before the battle to frighten the enemy, as the best preserved found in Tintignac (Gaul) in 2004 and which were decorated with a boar head or a snake head.

The interlace patterns that are often regarded as typical of "Celtic art" were characteristic of the whole of the British Isles, a style referred to as Insular art, or Hiberno-Saxon art. This artistic style incorporated elements of La Tène, Late Roman, and, most importantly, animal Style II of Germanic Migration Period art. The style was taken up with great skill and enthusiasm by Celtic artists in metalwork and illuminated manuscripts. Equally, the forms used for the finest Insular art were all adopted from the Roman world: Gospel books like the Book of Kells and Book of Lindisfarne, chalices like the Ardagh Chalice and Derrynaflan Chalice, and penannular brooches like the Tara Brooch and Roscrea Brooch. These works are from the period of peak achievement of Insular art, which lasted from the 7th to the 9th centuries, before the Viking attacks sharply set back cultural life.

In contrast the less well known but often spectacular art of the richest earlier Continental Celts, before they were conquered by the Romans, often adopted elements of Roman, Greek and other "foreign" styles (and possibly used imported craftsmen) to decorate objects that were distinctively Celtic. After the Roman conquests, some Celtic elements remained in popular art, especially Ancient Roman pottery, of which Gaul was actually the largest producer, mostly in Italian styles, but also producing work in local taste, including figurines of deities and wares painted with animals and other subjects in highly formalised styles. Roman Britain also took more interest in enamel than most of the Empire, and its development of champlevé technique was probably important to the later Medieval art of the whole of Europe, of which the energy and freedom of Insular decoration was an important element. Rising nationalism brought Celtic revivals from the 19th century.

The Coligny calendar, which was found in 1897 in Coligny, Ain, was engraved on a bronze tablet, preserved in 73 fragments, that originally was 1.48 metres (4 feet 10 inches) wide and 0.9 metres (2 feet 11 inches) high (Lambert p. 111). Based on the style of lettering and the accompanying objects, it probably dates to the end of the 2nd century. It is written in Latin inscriptional capitals, and is in Gaulish. The restored tablet contains 16 vertical columns, with 62 months distributed over 5 years.

French archaeologist J. Monard speculated that it was recorded by druids wishing to preserve their tradition of timekeeping in a time when the Julian calendar was imposed throughout the Roman Empire. However, the general form of the calendar suggests the public peg calendars (or parapegmata) found throughout the Greek and Roman world.

Tribal warfare appears to have been a regular feature of Celtic societies. While epic literature depicts this as more of a sport focused on raids and hunting rather than organised territorial conquest, the historical record is more of tribes using warfare to exert political control and harass rivals, for economic advantage, and in some instances to conquer territory.

The Celts were described by classical writers such as Strabo, Livy, Pausanias, and Florus as fighting like "wild beasts", and as hordes. Dionysius said that their:

manner of fighting, being in large measure that of wild beasts and frenzied, was an erratic procedure, quite lacking in military science. Thus, at one moment they would raise their swords aloft and smite after the manner of wild boars, throwing the whole weight of their bodies into the blow like hewers of wood or men digging with mattocks, and again they would deliver crosswise blows aimed at no target, as if they intended to cut to pieces the entire bodies of their adversaries, protective armour and all.

Polybius (2.33) indicates that the principal Celtic weapon was a long bladed sword which was used for hacking edgewise rather than stabbing. Celtic warriors are described by Polybius and Plutarch as frequently having to cease fighting in order to straighten their sword blades. This claim has been questioned by some archaeologists, who note that Noric steel, steel produced in Celtic Noricum, was famous in the Roman Empire period and was used to equip the Roman military. However, Radomir Pleiner, in The Celtic Sword (1993) argues that "the metallographic evidence shows that Polybius was right up to a point", as around one third of surviving swords from the period might well have behaved as he describes. In addition to these long bladed slashing swords, spears and specialized javelins were also used.

Polybius also asserts that certain of the Celts fought naked: "The appearance of these naked warriors was a terrifying spectacle, for they were all men of splendid physique and in the prime of life." According to Livy, this was also true of the Celts of Asia Minor.

Celts had a reputation as head hunters. Paul Jacobsthal says: "Amongst the Celts the human head was venerated above all else, since the head was to the Celt the soul, centre of the emotions as well as of life itself, a symbol of divinity and of the powers of the other-world." Writing in the first century BC, Greek historians Posidonius and Diodorus Siculus said Celtic warriors cut off the heads of enemies slain in battle, hung them from the necks of their horses, then nailed them up outside their homes. Strabo wrote in the same century that Celts embalmed the heads of their most esteemed enemies in cedar oil and put them on display. Roman historian Livy wrote that the Boii beheaded a defeated Roman general after the Battle of Silva Litana, covered his skull in gold, and used it as a ritual cup. Archaeologists have found evidence that heads were embalmed and displayed by the southern Gauls. In another example, at the southern Gaulish site of Entremont, there stood a pillar carved with skulls, within which were niches where human skulls were kept, nailed into position. Roquepertuse nearby has similar carved heads and skull niches. Many lone carved heads have been found in Celtic regions, some with two or three faces. Examples include the Mšecké Žehrovice Head and the Corleck Head.

Severed heads are a common motif in Insular Celtic myths, and there are many tales in which 'living heads' preside over feasts or speak prophecies. The beheading game is a motif in Irish myth and Arthurian legend, most famously in the tale Sir Gawain and the Green Knight, where the Green Knight picks up his own severed head after Gawain has struck it off. There are also many legends in Celtic regions of saints who carry their own severed heads. In Irish myth, the severed heads of warriors are called the mast or nuts of the goddess Macha.

Like other European Iron Age societies, the Celts practised a polytheistic religion and believed in an afterlife. Celtic religion varied by region and over time, but had "broad structural similarities", and there was "a basic religious homogeneity" among the Celtic peoples. Because the ancient Celts did not have writing, evidence about their religion is gleaned from archaeology, Greco-Roman accounts, and literature from the early Christian period.

The names of over two hundred Celtic deities have survived (see list of Celtic deities), although it is likely that many of these were alternative names, regional names or titles for the same deity. Some deities were venerated only in one region, but others were more widely known. According to Miranda Aldhouse-Green, the Celts were also animists, believing that every part of the natural world had a spirit.

The Celts seem to have had a father god, who was often a god of the tribe and of the dead (Toutatis probably being one name for him); and a mother goddess who was associated with the land, earth and fertility (Dea Matrona probably being one name for her). The mother goddess could also take the form of a war goddess as protectress of her tribe and its land. There also seems to have been a male celestial god—identified with Taranis—associated with thunder, the wheel, and the bull. There were gods of skill and craft, such as the pan-regional god Lugus, and the smith god Gobannos. Celtic healing deities were often associated with sacred springs, such as Sirona and Borvo. Other pan-regional deities include the horned god Cernunnos, the horse and fertility goddess Epona, the divine son Maponos, as well as Belenos, Ogmios, and Sucellos. Caesar says the Gauls believed they all descended from a god of the dead and underworld. Triplicity is a common theme in Celtic cosmology, and a number of deities were seen as threefold, for example the Three Mothers.

Celtic religious ceremonies were overseen by priests known as druids, who also served as judges, teachers, and lore-keepers. Other classes of druids performed sacrifices for the perceived benefit of the community. There is evidence that ancient Celtic peoples sacrificed animals, almost always livestock or working animals. It appears some were offered wholly to the gods (by burying or burning), while some were shared between gods and humans (part eaten and part offered). There is also some evidence that ancient Celts sacrificed humans, and some Greco-Roman sources claim the Gauls sacrificed criminals by burning them in a wicker man.

The Romans said the Celts held ceremonies in sacred groves and other natural shrines, called nemetons. Some Celtic peoples built temples or ritual enclosures of varying shapes (such as the Romano-Celtic temple and viereckschanze), though they also maintained shrines at natural sites. Celtic peoples often made votive offerings: treasured items deposited in water and wetlands, or in ritual shafts and wells, often in the same place over generations. Modern clootie wells might be a continuation of this.

Most surviving Celtic mythology belongs to the Insular Celtic peoples: Irish mythology has the largest written body of myths, followed by Welsh mythology. These were written down in the early Middle Ages, mainly by Christian scribes.

The supernatural race called the Tuatha Dé Danann are believed to represent the main Celtic gods of Ireland. Their traditional rivals are the Fomóire, whom they defeat in the Battle of Mag Tuired. Barry Cunliffe says the underlying structure in Irish myth was a dualism between the male tribal god and the female goddess of the land. The Dagda seems to have been the chief god and the Morrígan his consort, each of whom had other names. One common motif is the sovereignty goddess, who represents the land and bestows sovereignty on a king by marrying him. The goddess Brigid was linked with nature as well as poetry, healing and smithing.

Some figures in medieval Insular Celtic myth have ancient continental parallels: Irish Lugh and Welsh Lleu are cognate with Lugus, Goibniu and Gofannon with Gobannos, Macán and Mabon with Maponos, while Macha and Rhiannon may be counterparts of Epona.

In Insular Celtic myth, the Otherworld is a parallel realm where the gods dwell. Some mythical heroes visit it by entering ancient burial mounds or caves, by going under water or across the western sea, or after being offered a silver apple branch by an Otherworld resident. Irish myth says that the spirits of the dead travel to the house of Donn (Tech Duinn), a legendary ancestor; this echoes Caesar's comment that the Gauls believed they all descended from a god of the dead and underworld.

Insular Celtic peoples celebrated four seasonal festivals, known to the Gaels as Beltaine (1 May), Lughnasa (1 August), Samhain (1 November) and Imbolc (1 February).

The Roman invasion of Gaul brought a great deal of Celtic peoples into the Roman Empire. Roman culture had a profound effect on the Celtic tribes which came under the empire's control. Roman influence led to many changes in Celtic religion, the most noticeable of which was the weakening of the druid class, especially religiously; the druids were to eventually disappear altogether. Romano-Celtic deities also began to appear: these deities often had both Roman and Celtic attributes, combined the names of Roman and Celtic deities, or included couples with one Roman and one Celtic deity. Other changes included the adaptation of the Jupiter Column, a sacred column set up in many Celtic regions of the empire, primarily in northern and eastern Gaul. Another major change in religious practice was the use of stone monuments to represent gods and goddesses. The Celts had probably only created wooden cult images (including monuments carved into trees, which were known as sacred poles) before the Roman conquest.

While the regions under Roman rule adopted Christianity along with the rest of the Roman empire, unconquered areas of Ireland and Scotland began to move from Celtic polytheism to Christianity in the 5th century. Ireland was converted by missionaries from Britain, such as Saint Patrick. Later missionaries from Ireland were a major source of missionary work in Scotland, Anglo-Saxon parts of Britain, and central Europe (see Hiberno-Scottish mission). Celtic Christianity, the forms of Christianity that took hold in Britain and Ireland at this time, had for some centuries only limited and intermittent contact with Rome and continental Christianity, as well as some contacts with Coptic Christianity. Some elements of Celtic Christianity developed, or retained, features that made them distinct from the rest of Western Christianity, most famously their conservative method of calculating the date of Easter. In 664, the Synod of Whitby began to resolve these differences, mostly by adopting the current Roman practices, which the Gregorian Mission from Rome had introduced to Anglo-Saxon England.

Genetic studies on the limited amount of material available suggest continuity between Iron Age people from areas considered Celtic and the earlier Bell Beaker culture of Bronze Age Western Europe. Like the Bell Beakers, ancient Celts carried a substantial amount of Western Steppe Herders ancestry, which is derived from Yamnaya pastoralists who expanded westwards from the Pontic–Caspian steppe during the late Neolithic and the early Bronze Age and associated with the initial spread of Indo-European languages. This ancestry was particularly prevalent among Celts of Northwest Europe. Examined individuals overwhelmingly carry types of the paternal haplogroup R-M269, while the maternal haplogroups H and U are frequent. These lineages are associated with steppe ancestry. The spread of Celts into Iberia and the emergence of the Celtiberians is associated with an increase in north-central European ancestry in Iberia, and may be connected to the expansion of the Urnfield culture. The paternal haplogroup haplogroup I2a1a1a has been detected among Celtiberians. There appears to have been significant gene flow among Celtic peoples of Western Europe during the Iron Age. While the Gauls of southern France display genetic links with the Celtiberians, the Gauls of northern France display links with Great Britain and Sweden. Modern populations of Western Europe, particularly those who still speak Celtic languages, display substantial genetic continuity with the Iron Age populations of the same areas.

• List of ancient Celtic peoples and tribes
• Ethnic groups in Europe
• Celtic F.C., soccer club in Glasgow

• Ancient Celtic music – in the Citizendium
• Essays on Celtiberian topics – in e-Keltoi, University of Wisconsin, Madison
• Ancient Celtic Warriors in History
• Celts descended from Spanish fishermen, study finds
• Discussion – with academic Barry Cunliffe, on BBC Radio 4's In Our Time, 21 February 2002. (Streaming RealPlayer format)

Geography

• An interactive map showing the lands of the Celts between 800 BC and 305 AD.
• Detailed map of the Pre-Roman Peoples of Iberia (around 200 BC), showing the Celtic territories
• Map of Celtic lands Archived 25 February 2021 at the Wayback Machine

Organisations

• newworldcelts.org
• XIII. International Congress of Celtic Studies in Bonn