Celticization is a supposed mechanism of the spread of the Celts and the Celtic languages

Celtic languages

The Celtic languages are descended from Proto-Celtic, or "Common Celtic"; a branch of the greater Indo-European language family...

 in Iron Age Europe.

During the first millennium BC, the early Celts expanded from a core territory in Central Europe to Iberia

Celtiberians

The Celtiberians were Celtic-speaking people of the Iberian Peninsula in the final centuries BC. The group used the Celtic Celtiberian language.Archaeologically, the Celtiberians participated in the Hallstatt culture in what is now north-central Spain...

, the British Isles

Insular Celts

The Insular Celts are the speakers of Insular Celtic languages.-Pre-Celtic Britain:Little is known of the culture and language of pre-Celtic Britain, but remnants of the latter may remain in the names of some geographical features, such as the rivers Clyde, Tamar and Thames, whose etymology is...

 and later also the Balkans

Gallic invasion of the Balkans

Gallic groups, originating from the various La Tène chiefdoms, began a south-eastern movement into the Balkan peninsula from the 4th century BC. Although Celtic settlements were concentrated in the western half of the Carpathian basin, there were notable incursions, and settlements, within the...

, and are assumed to have "Celticized" earlier populations such as Illyrians

Illyrians

The Illyrians were a group of tribes who inhabited part of the western Balkans in antiquity and the south-eastern coasts of the Italian peninsula...

 and Thracians

Thracians

The ancient Thracians were a group of Indo-European tribes inhabiting areas including Thrace in Southeastern Europe. They spoke the Thracian language – a scarcely attested branch of the Indo-European language family...

.

Illyria and Pannonia

The Celticization in Pannonia  began as early as the 4th century BC.La Tene type finds are charakteristic in Pre-Roman Pannonia and are considered a marker to variations in the degree of Celticization.Among the Illyrian tribes some were Celticized to varying degrees(some completely) like the Pannoni and the Dalmatae

Dalmatae

The Dalmatae or Delmatae were an ancient people who inhabited the core of what would then become known as Dalmatia after the Roman conquest - now the eastern Adriatic coast in Croatia, between the rivers Krka and Neretva...

. A type of wooden oblong shield with an iron boss was introduced to Illyria

Illyria

In classical antiquity, Illyria was a region in the western part of the Balkan Peninsula inhabited by the Illyrians....

 from the Celts. Illyrian chiefs and kings wore bronze torques around their necks much like the Celts did.

By the 4th century BC the Veneti

Adriatic Veneti

The Veneti were an ancient people who inhabited north-eastern Italy, in an area corresponding to the modern-day region of the Veneto....

 had been so Celticized that Polybius

Polybius

Polybius , Greek ) was a Greek historian of the Hellenistic Period noted for his work, The Histories, which covered the period of 220–146 BC in detail. The work describes in part the rise of the Roman Republic and its gradual domination over Greece...

 wrote that the Veneti of the 2nd century BC were identical to the Celts except for language.

The Celts had two settlements that later became cities in Illyria

Illyria

In classical antiquity, Illyria was a region in the western part of the Balkan Peninsula inhabited by the Illyrians....

, namely Navissos and Segestica.In Thrace

Thrace

Thrace is a historical and geographic area in southeast Europe. As a geographical concept, Thrace designates a region bounded by the Balkan Mountains on the north, Rhodope Mountains and the Aegean Sea on the south, and by the Black Sea and the Sea of Marmara on the east...

 they had Serdica (modern Sofia

Sofia

Sofia is the capital and largest city of Bulgaria and the 12th largest city in the European Union with a population of 1.27 million people. It is located in western Bulgaria, at the foot of Mount Vitosha and approximately at the centre of the Balkan Peninsula.Prehistoric settlements were excavated...

, Bulgaria

Bulgaria

Bulgaria , officially the Republic of Bulgaria , is a parliamentary democracy within a unitary constitutional republic in Southeast Europe. The country borders Romania to the north, Serbia and Macedonia to the west, Greece and Turkey to the south, as well as the Black Sea to the east...

),Tylis

Tylis

Tylis or Tyle was a capital of a short-lived Balkan state mentioned by Polybius that was founded by Celts led by Comontorios in the 3rd century BC, after their invasion of Thrace and Greece in 279 BC. It was located near the eastern edge of the Haemus Mountains in what is now eastern Bulgaria...

, founded by Gauls

Gauls

The Gauls were a Celtic people living in Gaul, the region roughly corresponding to what is now France, Belgium, Switzerland and Northern Italy, from the Iron Age through the Roman period. They mostly spoke the Continental Celtic language called Gaulish....

, Dunonia, Singidunum

Singidunum

Singidunum is the name for the ancient city in Serbia which became Belgrade, the capital of Serbia. It was recorded that a Celtic tribe Scordisci settled the area in the 3rd century BC following the Gallic invasion of the Balkans. The Roman Empire conquered the area in 75 BC and later garrisoned...

 and Taurunum.

Many Celtic tribes or parts of Celtic tribes migrated to Illyria, Thrace and Dacia.

The gradual Celticization of all of Pannonia took place in the 3rd century BC.Names become Celtic as see in Roman times and Celts have established control north of the Save and south and west of the Danube.In the western half and west of Pannonia the Pre-Celtic language disappeared. By the first half of the 1st century BC the language of the Illyrians

Illyrians

The Illyrians were a group of tribes who inhabited part of the western Balkans in antiquity and the south-eastern coasts of the Italian peninsula...

 in Northern Dalmatia

Dalmatia

Dalmatia is a historical region on the eastern coast of the Adriatic Sea. It stretches from the island of Rab in the northwest to the Bay of Kotor in the southeast. The hinterland, the Dalmatian Zagora, ranges from fifty kilometers in width in the north to just a few kilometers in the south....

 was completely Celticized.There is an abundance of Celt

Celt

The Celts were a diverse group of tribal societies in Iron Age and Roman-era Europe who spoke Celtic languages.The earliest archaeological culture commonly accepted as Celtic, or rather Proto-Celtic, was the central European Hallstatt culture , named for the rich grave finds in Hallstatt, Austria....

ic names in Illyria

Illyria

In classical antiquity, Illyria was a region in the western part of the Balkan Peninsula inhabited by the Illyrians....

 sometimes making the Illyrian ones seem few.Those parts of Pannonia that had not been conquered by the first Celtic invasion were already Celticized by the beginning of the 3rd century BC.The Dalmatae

Dalmatae

The Dalmatae or Delmatae were an ancient people who inhabited the core of what would then become known as Dalmatia after the Roman conquest - now the eastern Adriatic coast in Croatia, between the rivers Krka and Neretva...

 had been Celticized by the 3rd century BC.In the region of Pannonia

Pannonia

Pannonia was an ancient province of the Roman Empire bounded north and east by the Danube, coterminous westward with Noricum and upper Italy, and southward with Dalmatia and upper Moesia....

as a Roman province Celticization had almost completely eradicated Illyrian culture.