The starting-point for any history of medieval and modern Greek must be the κοινὴ διάλεκτοζ or common Greek of the Hellenistic world. This form of the language was from the first the vehicle of communication at all levels in the new Greek cities which were founded between the Aegean coast of Asia Minor and the plains of the Punjab, from the Syr-Darya in the North to the island of Sokotra in the South. In old Greece it rapidly became the official language of administration and more slowly ousted the old dialects as a general means of communication. At the same time it became the universal language of prose literature, apart from certain highly self-conscious groups which retained a special linguistic form of their own, e.g. the doctors who wrote in the Ionic of the Hippocratic corpus, and the Pythagorean philosophers who wrote in the Doric of Southern Italy. Poetry continued to be written in the traditional linguistic forms, though these were more and more affected by the κοινὴ διάλεκτοζ.

Koine Greek came into being suddenly, in response to a sudden and radical transformation of the Near Eastern world, whereby Greek became the language of culture, and in some cases the mother-tongue, of men and societies over a vast area. There are no precise parallels for such a rapid and dramatic extension of a language. Both the expansion of Arabic after the great conquests of Islam and the development of English into a world language took place in different circumstances and against quite different backgrounds.