Baltic and Slavic languages share several linguistic traits not found in any other Indo-European branch,[3] which points to a period of common development and origin.

[5] While the notion of a Balto-Slavic unity was previously contested largely due to political controversies, there is now a general consensus among academic specialists in Indo-European linguistics that Baltic and Slavic languages comprise a single branch of the Indo-European language family, with only some minor details of the nature of their relationship remaining in contention.

A few are more intent on explaining the similarities between the two groups not in terms of a linguistically "genetic" relationship, but by language contact and dialectal closeness in the Proto-Indo-European period.

The early Indo-Europeanists Rasmus Rask and August Schleicher (1861) proposed a simple solution: From Proto-Indo-European descended Balto-German-Slavonic language, out of which Proto-Balto-Slavic (later split into Proto-Baltic and Proto-Slavic) and Germanic emerged.

Antoine Meillet (1905, 1908, 1922, 1925, 1934), a French linguist, in reaction to Brugmann's hypothesis, propounded a view according to which all similarities of Baltic and Slavic occurred accidentally, by independent parallel development, and that there was no Proto-Balto-Slavic language.

[10] Even though some linguists still reject a genetic relationship, most scholars accept that Baltic and Slavic languages experienced a period of common development.

[11][12][13][14] Gray and Atkinson's (2003) application of language-tree divergence analysis supports a genetic relationship between the Baltic and Slavic languages, dating the split of the family to about 1400 BCE.

[15] The traditional division into two distinct sub-branches (i.e. Slavic and Baltic) is mostly upheld by scholars who accept Balto-Slavic as a genetic branch of Indo-European.

[34] Hydronymic evidence suggests that Baltic languages were once spoken in much wider territory than the one they cover today, all the way to Moscow, and were later replaced by Slavic.