Illyrians has come to refer to a broad, ill-defined "Indo-European group of peoples who inhabited the western Balkans (Illyria, roughly from central Albania to southern Pannonia) and even possibly Messapia in Southern Italy (if the Messapian language is to be considered an Illlyrian dialect). Illyrians were part of the Hallstatt culture.
In theory, Illyrians are defined as speakers of the Illyrian languages, but since the latter is practically unknown, this entails the danger of a circular definition. The existence of a broad "Illyrian" ethnic identity in the past is uncertain, and some argue that the ethnonym Illyrioi came to be applied to this large group of peoples by the ancient Greeks, Illyrioi having perhaps originally designated only a single people that came to be widely known to the Greeks due to proximity.
Indeed, such a people known as the Illyrioi are supposed to have occupied a small and well-defined part of the south Adriatic coast, around Skadar Lake astride the modern frontier between Albania and Montenegro. The name may then have expanded and come to be applied to ethnically different peoples such as the Liburni, Delmatae, Iapodes, or the Pannonii.
Messapian (also known as Messapic) is an extinct Indo-European language of South-eastern Italy, once spoken in Messapia (modern Salento). It was spoken by the three Iapygian tribes of the region: the Messapians, the Daunii and the Peucetii. If Messapian is considered an Illyrian dialect, the vast majority of our knowledge of Illyrian is based on Messapian. The non-Messapic testimonies of Illyrian are too fragmentary to be certain whether Messapian should be considered part of Illyrian proper, but it is widely thought that Messapian was in some way related to Illyrian.
All these languages were likely extinct by the 6th century, though the Albanian language may represent a remote descendant of Thraco-Illyrian dialects that survived in remote areas of the Balkans during the Middle Ages. This would have happened along the boundary of Latin and Greek linguistic influence (the Jireček Line). Not enough is known of the ancient language to either prove or disprove this hypothesis (see Origin of Albanians).
When the Proto-Illyrians became a distinct group remains unclear. The process may have begun as early as the Eneolithic (the latest phase of the Stone Age). It is hypothesized that in the Eneolithic period invading Indo-European groups mingled with indigenous pre-Indo-European groups, resulting in the formation of the principal tribal groups, based upon their uses of the Paleo-Balkan languages (Illyrians, Thracians, and others). Many Illyrian tribes are not considered Illyrian anymore but Venetic.
A. Benac and B. Čović, archaeologists from Sarajevo, hypothesize that during the Bronze Age there took place a progressive Illyrianization of peoples dwelling in the lands between the Adriatic and the Sava river. In contrast to an ethnogenesis in the Balkans, another (older) school of scholars maintains the theory of an Illyrian invasion, which involves a great movement of Illyrian tribes from the lowlands of central Europe (modern Hungary), towards southeastern Europe and the Balkan peninsula. The Illyrian invasion is estimated to have occurred around the 13th century BC. The numerous Thracian names in Illyria have led many scholars to believe that the region was originally inhabited by Thracians, who were either displaced or submitted to the Illyrian invaders. The Illyrians were most likely in turn pushed eastwards by Celtic or Germanic tribes from the northwest. According to this theory, the Illyrian invasion most likely caused the Thracian expansion to the east, the movement of the Greeks to the south and the Phrygian migration from Thrace into central Asia Minor. The last event may have created the conditions for the Achaean Greeks to colonize the coast of Asia Minor and the Dorians to start their invasion.
During the 7th century BC, when bronze was replaced by iron, the Illyrians became an ethnic group with a distinct culture and art form, and only jewelry and art objects were still made out of bronze. Different Illyrian tribes appeared, under the influence of the Halstat cultures from the north, and they organized their regional centers. The cult of the dead played an important role in the lives of the Illyrians, which is seen in their carefully made burials and burial ceremonies, as well as the richness of the burial sites. In the northern parts of the Balkans, there existed a long tradition of cremation and burial in shallow graves, while in the southern parts, the dead were buried in large stone, or earth tumuli (natively called gromile) that in Herzegovina were reaching monumental sizes, more than 50 meters wide and 5 meters high. The Japodian tribe (found from Istria in Croatia to Bihać in Bosnia) have had an affinity for decoration with heavy, oversized necklaces out of yellow, blue or white glass paste, and large bronze fibulas, as well as spiral bracelets, diadems and helmets out of bronze. Small sculptures out of jade in form of archaic Ionian plastic are also characteristically Japodian. Numerous monumental sculptures are preserved, as well as walls of citadel Nezakcij near Pula, one of numerous Istrian cities from Iron Age.
The Illyrians formed several kingdoms in the central Balkans, and the first known Illyrian king was Bardyllis. Illyrian kingdoms were often at war with ancient Macedonia, and the Illyrian pirates were also a significant danger to neighbouring peoples. At the delta of Neretva, there was a strong Hellenistic influence on the Illyrian tribe of Daors. Their capital was Daorson located in Ošanići near Stolac in Herzegovina, which became the main center of classical Illyrian culture. Daorson, during the 4th century BC, was surrounded by megalithic, 5 meter high stonewalls (large as those of Mycenae in Greece), composed out of large trapeze stones blocks. Daors also made unique bronze coins and sculptures. The Illyrians even conquered Greek colonies on the Dalmatian islands. Queen Teuta of Issa (today the Vis (island)) was famous for having waged wars against the Romans. Ultimately, the Romans subdued the Illyrians during the 1st century BC. Illyrian territories would later become provinces of the Roman Empire and the Byzantine Empire.
After the Roman conquest, the Illyrians were granted civil rights by the Constitutio Antoniniana issued in 212. Moreover, Rome recruited Illyrian soldiers to guard its borders from barbarian tribes. Their squads grew in number to such an extent that the Illyrian military began to play an important part in Roman political life, even ascending the imperial throne at certain points. In the course of over a century, seven Illyrian-born emperors ruled in succession. The province of Illyricum stretched from the Drilon River in modern Albania to Istria (Croatia) in the west and the Sava River (Croatia) in the north. Its capital was located at Salonae near modern Split in Croatia. After a revolt of Pannonians and Dalmatians (i.e. the rebellion of Bato) was crushed in 9, the province of Illyricum was dissolved, and its lands were divided between the new provinces of Pannonia in the north and Dalmatia in the south. In 297 CE, Emperor Diocletian, carried out administrative reforms in the Roman Empire by constituting prefectures, dioceses and provinces. In conformity with this reorganisation, the territory was additionally divided. The southern part of Dalmatia was cut off and became the province of Praevalitana, with Shkodra (Shkodër) as its administrative centre. Other provinces entailed Epirus Nova, with Dyrrachium as its capital, and Epirus Vetus, with its central city at Nikopolis. The latter two were part of the Macedonian diocese. The dioceses Pannonia, Noricum, Moesia Superior and Macedonia with the rest of Greece were constituent parts of the prefecture of Illyricum, which comprised part of the Balkans from 318 to 379 AD. Liburnia was also one of the provincia Dalmatiarum located north from the river Krka including Skradin. However, Liburnia had sometimes possessed a status of being in a separate administrative-territorial unit. Analysis of the archaeological material from antiquity has shown that the process of Romanization was selective. Urban centres, both coastal and inland, were almost totally Romanised. Although the Illyrians were subject to a strong process of aculturisation, they continued to speak their own native language, respect their own gods, preserve their traditions and adhere to their own socio-political tribal organization, which was only in some necessities adapted to Roman administration and political structures.
The term Illyrians was utilized in late medieval texts such as in Mazaris' Journey to Hades (a work written by Byzantine author Mazaris between January 1414 and October 1415). In Mazaris' case, the term was used to designate "Albanians" (i.e. Arvanites).
The term was revived again during the Habsburg Monarchy, but it was designated towards South Slavs. This association was based on the opinion that the South Slavs were descendants of Slavicized Illyrians. When Napoleon conquered part of the South Slavic lands in the beginning of the 19th century, these areas were named after ancient Illyrian provinces. Under the influence of Romantic nationalism, a self-identified "Illyrian movement" (Ilirski pokret) in the form of a Croatian national revival, opened a literary and journalistic campaign that was initiated by a group of young Croatian intellectuals during the years of 1835-1849. This movement, under the banner of Illlyrism, aimed to create a Croatian national establishment under Austro-Hungarian rule, through linguistic and ethnic unity among South Slavs. It was repressed by the Habsburg authorities after the failed Revolutions of 1848.