The purpose of the article is to reveal the features of the study of relict Paleo-Balkan languages in their connection with other Indo-European languages in linguistics of the second half of the XIX century – the beginning of the XXI century. It is noted that when identifying a group of Paleo-Balkan languages, a single criterion has not yet been clearly established: both areal and genetic principles are involved. At the same time the material of Paleo-Balkan languages is not fully taken into account in the genealogical classification of Indo-European languages; if Thracian, Phrygian, and Illyrian languages are included in this classification as separate groups, then the place of old Macedonian, “Pelasgian”, and other Paleo-Balkan languages in the classification is not defined. In addition, the traditional genealogical classification does not take into account the special proximity of Paleo-Balkan languages to Armenian, Albanian and Ancient Greek. The discovery and study of Paleo-Balkan languages are briefly described. The connections of Paleo-Balkan languages with Albanian, Armenian, Ancient Greek, Hittite, Luvian, Baltic, Germanic, Slavic and Indo-Iranian languages are revealed. The author of the article focuses on the fact that Paleo-Balkan languages are mainly reconstructed at the phonological, lexical, derivational levels (to a lesser extent – at the morphological and syntactic levels), although due to the limited and uneven corresponding data, such reconstruction is also limited, and in some cases contradictory. From the point of view of Indo-Europeanists, lexical elements of Paleo-Balkan origin are recorded in modern Albanian, Greek, Bulgarian, Romanian and Armenian. The article emphasizes that Paleo-Balcanistics provided significant factual material important for substrate theory and contributed to the development of the linguistic stratigraphy method.