Pan-Illyrian hypotheses or pan-Illyrian theories were proposed in the first half the twentieth century by philologists who thought that traces of Illyrian languages could be found in several parts of Europe, outside the Balkan area. Such ideas have been collectively termed pan-Illyrianism or pan-Illyrism.
The term Thraco-Illyrian refers to a hypothesis according to which the Daco-Thracian and Illyrian languages comprise a distinct branch of Indo-European. Thraco-Illyrian is also used as a term merely implying a Thracian-Illyrian interference, mixture or sprachbund, or as a shorthand way of saying that it is not determined whether a subject is to be considered as pertaining to Thracian or Illyrian. Downgraded to a geo-linguistic concept, these languages are referred to as Paleo-Balkan.
The linguistical hypothesis was especially current in the early 20th century, but after the 1960s it was seriously called into question. New publications argued that no strong evidence for Thraco-Illyrian exists, and that the two language-areas show more differences than correspondences (Vladimir Georgiev, Ivan Duridanov, Eric Hamp, et al.). It has also been pointed out that the onomastic studies carried out in the 20th century were conducted through pan-Thracian and pan-Illyrian theoretical approaches, which are considered outdated in current historical linguistics.