The characteristics Indo-European languages share with respect to vocabulary and grammar have led many scholars to postulate that they are all descended from an original parent language, called Proto-Indo-European, which is believed to have been spoken some time before 4000 B.C., perhaps before 8000 B.C. or earlier. Since there are no written records of Proto-Indo-European, it apparently was in use before writing was known to its speakers. Even its existence is an assumption, although a plausible one and the only really satisfactory explanation of the common features of the modern Indo-European languages. There has been much speculation as to the region where the speakers of Proto-Indo-European first lived and the nature of their culture, but nothing definite is known. One theory of the origin of the individual Indo-European languages suggests that as the ancient speakers of Proto-Indo-European migrated or moved away from each other, losing contact, their language broke up into a number of tongues. These tongues later also split up still further, eventually giving rise to the many modern Indo-European languages. For a classification of Indo-European subfamilies, groups, subgroups, and individual languages, see the table entitled The Indo-European Family of Languages. By studying the vocabulary and grammar of the various daughter languages of which there are records, scholars have tried to reconstruct Proto-Indo-European and infer some of its characteristics. It appears to have been highly inflected in a distinctive way. Apparently, it also had three genders (masculine, feminine, and neuter) for nouns, pronouns, and adjectives; eight cases for the noun; agreement between adjectives and nouns; and a free accent (i.e., one that could be placed on any syllable).

The descendant languages have all tended to discard to a greater or lesser extent these features of the mother tongue and to become simplified. For example, they substitute increasingly the use of word order and prepositions for inflections to indicate the relationships of words in a sentence. There also exists among the Indo-European languages a similarity of basic words (such as words denoting kinship, numerals, and parts of the body) that points to a common origin. Different forms of writing for the various Indo-European languages used both in ancient and modern times include cuneiform, hieroglyphics, and a number of alphabets, among them the Devanagari, Greek, Roman, and Arabic scripts.

See articles on many of the Indo-European subfamilies, groups, and languages.