Speakers of modern Baltic languages are generally concentrated within the borders of Lithuania and Latvia, and in emigrant communities in the United States, Canada, Australia and former Soviet states. Historically the languages were spoken over a larger area: West to the mouth of the Vistula river in present-day Poland, at least as far East as the Dniepr river in present-day Belarus, perhaps even to Moscow, perhaps as far south as Kiev. Key evidence of Baltic language presence in these regions is found in hydronyms (names of bodies of water) in the regions that are characteristically Baltic. Use of hydronyms is generally accepted to determine the extent of these cultures' influence, but not the date of such influence. Historical expansion of the usage of Slavic languages in the South and East, and Germanic languages in the West reduced the geographic distribution of Baltic languages to a fraction of the area which they had formerly covered.
With the establishment of a German state in Prussia, and the relocation of much of the Baltic Prussian population in the 13th century, Prussians began to be assimilated, and by the end of the 17th century, the Prussian language had become extinct.
During the years of the Polish-Lithuanian Commonwealth (1569-1795), official documents were written in Polish, Ruthenian and Latin, with Lithuanian being mostly an oral language, with small quantities of written documents.
Linguists disagree regarding the relationship of the Baltic languages to other languages in the Indo-European family. Such relationships are discerned primarily by the Comparative method, which seeks to reconstruct the chronology of the languages' divergence from each other in phonology and lexicon. Language kinship is generally determined by the identification of linguistic innovations that are held in common by two languages or groups.
Several of the extinct Baltic languages have a limited or nonexistent written record, their existence being known only from the records of ancient historians and personal or place names; all of the languages in the Baltic group (including the living ones) were first written down relatively late in their probable existence as distinct languages. These two factors combined with others have obscured the history of the Baltic languages, leading to a number of theories regarding their position in the Indo-European family.
According to most linguists, the Baltic languages show closest relationship with the Slavic languages, and are commonly reconstructed to have passed through common Proto-Balto-Slavic stage, during which numerous Common Balto-Slavic lexical, phonological, morphological and accentological isoglosses developed. Comparative Balto-Slavic accentology is one of the most active branches of Indo-European studies nowadays, with numerous mysteries still waiting to be solved. Even the commonly accepted facts - such as Winter's law, identical reflexes of Proto-Indo-European syllabic sonorants or development of Balto-Slavic mobile paradigms - have many intricate problems in their formulations.
Most linguists agree however that Baltic languages do not represent a genetic node in Indo-European family. There are virtually no non-trivial isoglosses that connect Baltic languages with respect to Proto-Indo-European and leave Slavic languages aside; West and East Baltic languages seem to differ from each other as much as each of them differs from Proto-Slavic, and all major isoglosses that differ Slavic from Baltic that are usually mentioned are either Proto-Indo-European archaisms preserved in Baltic or later innovations in Slavic that occurred during Common Slavic period, and not some "Common Baltic" innovations. Thus, there was most likely no "Proto-Baltic" stage, and Baltic languages would thus represent an archaic remnant of former Balto-Slavic dialect continuum, the last Proto-Indo-European branch to finally split around 1500-1000 BCE.