Middle Iranian is represented not only by Middle Persian and the closely related Parthian language but also by several Central Asian tongues. Although it is known chiefly through inscriptions of the early kings of the following Sassanian period, Parthian declined when Sassanian power expanded. During the Arsacid period, however, it influenced Persian. Although much of the Middle Persian literature was translated into Arabic, the bulk of its writings was lost during Islamic times. Sogdian produced a body of Christian, Buddhist, and secular literature, and Saka's Khotanese dialect was the vehicle of an important Buddhist literature.
Most Khwarezmian texts are from the post-Islamic period.
Bactrian is known only in a few recently discovered inscriptions in Afghanistan. Modern Persian had developed by the 9th century. It is a continuation of an area-wide standard language that had considerable Parthian and Middle Persian elements, with additional influences from other Iranian languages. Its grammar is simpler than that of Middle Persian, and it has absorbed a vast Arabic vocabulary.