Paleo-Balkan studies are obscured by the scarce attestation of these languages outside of Ancient Greek and, to a lesser extent, Messapic and Phrygian. Although linguists consider each of them to be member of the Indo-European family of languages, the internal relationships are still debated.
Illyrian is a group of reputedly Indo-European languages whose relationship to other Indo-European languages as well as to the languages of the Paleo-Balkan group, many of which might be offshoots of Illyrian, is poorly understood due to the paucity of data and is still being examined. The Illyrian languages are often considered to be centum dialects, but this is not confirmed as there are hints of satemization. Today, the main source of authoritative information about the Illyrian language consists of a handful of Illyrian words cited in classical sources, and numerous examples of Illyrian anthroponyms, ethnonyms, toponyms and hydronyms.
A grouping of Illyrian with Messapian has been proposed for about a century, but remains an unproven hypothesis. The theory is based on classical sources, archaeology, as well as onomastic considerations. Messapian material culture bears a number of similarities to Illyrian material culture. Some Messapian anthroponyms have close Illyrian equivalents.
A grouping of Illyrian with Venetic and Liburnian, once spoken in northeastern Italy and Liburnia respectively, is also proposed. The consensus now is that Illyrian was quite distinct from Venetic and Liburnian, but a close linguistic relation has not been ruled out and is still being investigated.
The place of Paeonian remains unclear. Not much has been determined in the study of Paeonian, and some linguists do not recognize a Paeonian area separate from Illyrian or Thracian. The classification of Ancient Macedonian and its relationship to Greek are also under investigation, with solid sources pointing that Ancient Macedonian is in fact a variation of Doric Greek, but also the possibility of being only related through the local sprachbund.
Phrygian, on the other hand, is considered to have been most likely closely related to Greek.
Modern studies has shown that assertions about the proximity of Greek and Phrygian with Armenian are not confirmed in the language material, and that the Armenian language is as close to Greek and Phrygian as it is to Indo-Iranian.
The Albanian language is considered by current linguistic consensus to have developed from one of the non-Greek, ancient Indo-European languages of the region. For more historical and geographical reasons than specifically linguistic ones, the widespread claim is that Albanian is the modern descendant of Illyrian, spoken in much the same region in classical times. Alternative hypotheses hold that Albanian may have descended from Thracian or Daco-Moesian, other ancient languages spoken farther east than Illyrian. Not enough is known of these languages to completely prove or disprove the various hypotheses.
^Boardman, John; Edwards, I. E. S.; Hammond, N. G. L.; Sollberger, E. (1970). The Cambridge Ancient History. Cambridge University Press. pp. 876. ISBN978-0-521-22496-3. Such a lexical difference would, however, be hardly enough evidence to separate Daco-Moesian from Thracian [...]
^Georgiev, Vladimir Ivanov (1977). Trakite i technijat ezik [Thacian and their Languages] (in Bulgarian). Bulgarian Academy of Sciences. p. 282.
^Price, Glanville (2000). Encyclopedia of the Languages of Europe. Wiley-Blackwell. ISBN0-631-22039-9.CS1 maint: ref=harv (link), p. 120
^ abKaticic 2012, p. 184: "And yet we know that it is the continuation of a language spoken in the Balkans already in ancient times. This has been proved by the fact that there are Ancient Greek loan words in Albanian".
^Wilkes, J. J. The Illyrians, 1992,ISBN0631198075, p. 183,"We may begin with the Venetic peoples, Veneti, Carni, Histri and Liburni, whose language set them apart from the rest of the Illyrians...."
^Cf. Paglia, Sorin (2002),"Pre-Slavic and Pre-Romance Place-Names in Southeast Europe." 'Proceedings of the 8th International Congress of Thracology', Sofia, Bulgarian Institute of Thracology – Europa Antiqua Foundation - Bulgarian Academy of Sciences, I, 219–229, who states: "According to the available data, we may surmise that Thracian and Illyrian were mutually understandable, e.g. like Czech and Slovak, in one extreme, or like Spanish and Portuguese, at the other."
^Vladimir Georgiev (1960), Raporturile dintre limbile dacă, tracă şi frigiană, "Studii Clasice" Journal, II, 1960, 39-58.
Crossland, R.A.; Boardman, John (1982). "Linguistic problems of the Balkan area in the late prehistoric and early Classical period" in The Cambridge Ancient History Volume 3, Part 1. Cambridge University Press. ISBN978-0-521-22496-3.CS1 maint: ref=harv (link)