Middle Persian, the major form of which is called Pahlavi, was the official language of the Sāsānians (224–651 ce). The most important of the Middle Persian inscriptions is that of Shāpūr I (d. 272 ce), which has parallel versions in Parthian and Greek. Middle Persian was also the language of the Manichaean and Zoroastrian books written during the 3rd to the 10th century ce.

The extant literature of the Zoroastrian books is much more extensive than that of the Manichaean texts, but the latter have the advantage of having been recorded in a clear and unambiguous script. Moreover, the Middle Persian of the Zoroastrian books does not simply represent the spoken language of the writers of the 9th-century Zoroastrian texts. It is probable that they spoke early Modern Persian and that their speech often impinged upon their writing but that they strove to write the Middle Persian of several centuries earlier as it was attested in the inscriptions of the early Sāsānian dynasty when Middle Persian was the koine. By contrast, in the case of Manichaean Middle Persian, some texts survive unchanged from the 3rd century ce, the time of the Persian teacher Mani himself (216–274 ce).

Very little Parthian survives from the pre-Sāsānian period. A large number of Parthian ostraca (inscribed pottery fragments) from the 1st century bce were discovered at Nisa near modern Ashkhabad, but they are inscribed in ideographic Aramaic (i.e., Aramaic writing that uses Aramaic words as symbols to represent Parthian words). Dating before the 3rd century are a document from Hawrāman, some coin legends, and a dated grave stela.

The most copious and important material in Parthian is the work of the Sāsānian kings of the 3rd century, who added a Parthian version to their inscriptions—Ḥājjīābād, Naqsh-e Rustam (Ka’be yi Zardusht), and Paiküla. A few decades later Parthian disappeared as a result of the rise of the Sāsānians and the predominance of their native tongue, Middle Persian. Manichaean Parthian of the 3rd century was preserved as a church language in Central Asia.

The oldest surviving Sogdian documents are the so-called Ancient Letters found in a watchtower on the Great Wall of China, west of Dunhuang, and dated at the beginning of the 4th century ce. Most of the religious literature written in Sogdian dates from the 9th and 10th centuries. The Manichaean, Buddhist, and Christian Sogdian texts come mainly from small communities of Sogdians in the Tulufan (Turfan) oasis and in Dunhuang. From Sogdiana itself there is only a small collection of documents from Mount Mugh in the Zarafshān region, mainly the business correspondence of a minor Sogdian king, Dewashtich, from the time of the Arab conquest about 700.

The relationship of the various forms of Sogdian to one another has not yet been sufficiently investigated, so that it is not clear whether different dialects are represented by the extant material or whether the differences can be accounted for by reference to other relevant factors, such as differences of script, period, subject, style, or social milieu. The importance of social milieu can be seen by comparing the elegant Manichaean literature directed to the court with the more vulgar language of the Christian literature directed to the lower classes.

Of the Saka dialect known as Tumshuq very little has survived, and despite its evidently close relationship to the much better known Khotanese dialect, full interpretation has proved difficult. Knowledge of Khotanese is more firmly based on a substantial corpus of material, including extensive bilingual texts. Although the chronological range of the extant Khotanese material is limited to only a few centuries, probably the 7th to the 10th, a rapid development of the language is apparent. At the phonological level, most noticeable is the loss of syllables between the older and later stages of the language. Thus, hvatana- “Khotanese” at the oldest stage is successively weakened to hvatäna-, hvaṃna-, hvana-, hvaṃ. At the morphological level, most striking is the tendency to simplify the case endings and even to replace them by analytical expressions, constructions of two or more words. Thus, Late Khotanese has rakṣaysā hīya rāde “kings of the rākṣasas,” whereas Old Khotanese would have rakṣaysänu rrunde. The Old Khotanese -änu ending is unmistakably genitive plural, but the Late Khotanese -ā is merely a general oblique plural ending and has been reinforced by hīya “own,” used to mean “of.”

Hotan was a great centre of Buddhism during the 1st millennium ce, and all the surviving literature in Khotanese is either Buddhist or coloured by Buddhism. Even in business documents and official letters the Buddhist background is usually not difficult to discern. It can scarcely be coincidental that the Buddhist literature of Hotan, flourishing so vigorously during the 10th century, ended abruptly with the Muslim conquest at the beginning of the 11th.

Little survives of Bactrian and Scytho-Sarmatian. Knowledge of Bactrian is based almost entirely on a single inscription of 25 lines from Āteshkadeh-ye Sorkh Kowtal in northern Afghanistan. Even less is known of Scytho-Sarmatian.

Little is also known of Old Khwārezmian; that is, Khwārezmian written in the indigenous Khwārezmian script. Apart from a few coin legends and inscriptions on silver vessels, the material that survives consists of inscriptions of the 2nd century ce from Topraq-qalʿah (Toprakkala) and of the 7th from Toqqalʿah, archaeological sites in Uzbekistan. Much more is known of Late Khwārezmian, written in the Arabic script. That material is found mainly in two Arabic works, the 13th-century fiqh work of Mukhtār al-Zāhidī, called the Qunyat almunyah, and the Arabic dictionary Muqaddimat al-Adab of Abu al-Qāsim Maḥmūd ibn ʿUmar al-Zamakhsharī (1075–1144), of which a manuscript glossed in Khwārezmian was found.

Modern Iranian

Of the modern Iranian languages, by far the most widely spoken is Persian, which, as already indicated, developed from Middle Persian and Parthian (with elements from other Iranian languages such as Sogdian) as early as the 9th century ce. Since then, it has changed little except for acquiring an increasing proportion of loanwords, mainly from Arabic. Persian has been a literary language since the 9th century, and there is an increasing awareness of the continuity of its literary tradition with the earlier periods.

As the national language of Iran in succession to Middle Persian, it has for centuries strongly influenced the other Iranian languages, especially on Iranian territory. In fact, it seems likely that, with the increase of modern methods of communication, Persian will eventually supplant entirely most of the other languages and dialects. Against this trend stand only Kurdish and Balochi, the speakers of which tend to regard their languages as an expression of their particular identities. Nevertheless, even Kurdish and Balochi have been and continue to be strongly influenced by Persian.

Outside Iran the situation is rather different. In Afghanistan the first national language is Pashto, even though Persian is the official second language. Pashto became the official language by royal decree in 1936, and literary activity has been encouraged by the Pashto Ṭolana (Pashto Society) of Kabul. During the Soviet period both Ossetic and Tajik received official encouragement; nevertheless, both languages were displaced by the Russian language as the language of administration.

Other languages also compete with Ossetic and Tajik. Though it has a large body of folk epics, Ossetic became a literary language only in the second half of the 19th century. By contrast, the neighbouring Georgian has a still flourishing ancient literary tradition dating back to the 5th century ce and has many more speakers. Tajik, on the other hand, has a lifeline through its close connection with Persian, but it too has been retreating before Uzbek, an unrelated language of the Turkic group.