The sources on ancient Iranian religion, though limited, consist of textual and material sources. The textual sources are both Iranian and non-Iranian.
An important Iranian source is the Avesta, which are Zoroastrian sacred scriptures made in the Avestan. This is considered the principal source of knowledge on ancient Iranian religions. It is a collection of several texts that seems to have been over a large span of time by a variety of authors. These texts have been subjected to editings and redactions throughout their development. It is now the only extant fragment of what remained in the 9th century AD of the Avesta compiled in the Sasanian Empire by Khosrow I (6th century). Summaries of its content reveal that it was a huge collection containing texts not only in Avestan, but also in Pahlavi, which was the language of Zoroastrianism in the Sasanian Empire. Though the existing Avesta is dated quite recently, it contains information that is considerably older. The Gathas ("Songs") of the Prophet Zoroaster and much of the Yashts are considered among the oldest. The Gathas includes expressions of the religious vision of Zoroaster, which in many ways is a reinterpretation of the ancient Iranian religious principles. The Yashts are a collection of verses dedicated to various deities. These verses are mostly related to Zoroastrian terminology and ideas, but have little relation to anything specifically Zoroastrian. The gods invoked are basically the pre-Zoroastrian gods of the Iranian peoples. There is little agreement on when Zoroaster lived, but most scholars agree that he lived somewhere between 1200 and 600 BC. Dating the Yashts is similarly difficult, but it is likely that they were redacted (not necessarily composed) initially in the 5th-century BC.
The 5th-century BC Greek
is an important source on ancient Iranian religion
Another Iranian source are royal inscriptions of the Achaemenid Empire made in the Old Persian (with Akkadian, Aramaic and Elamite translations). These inscriptions, in particular those of Darius I and his son Xerxes I, contain many references to religion. The fact that these are fixed in time and place make them particularly useful.
Except from the Achaemenid inscriptions, there is no evidence that the Iranian religious compositions were written until the late Parthian or Sassanid period. This makes ancient Iranian religion the only major religion of the Middle East which has no written texts in the ancient period. The religious information was rather oral both in composition and transmission.
The non-Iranian sources are mainly Greek. The most important Greek source is Herodotus. Some of the Greek information on ancient Iranian religion is however unreliable. This is either because it is based on outright wrong information or based on misunderstandings.
The historical reconstruction of ancient Vedic literature is also an important source. The earliest religious texts of the related Indo-Aryan peoples are indispensable for reconstructing the historical development of the ancient Iranian religion. The most important of these texts in this regard is the Rigveda. It is composed of more than 1,000 hymns dedicated to various deities. The Rigveda can be dated to the period between 1700 and 1100 BC.
Material sources are rather limited and mostly confined to western Iran. The remains of Achaemenid architecture are the most important of these material sources. They provide a mass of evidence of imperial articulation of religious symbols and indicate a significant dependence on Middle Eastern precedents.