All the Yashts as they survive are in Younger Avestan, but that shared surface language conceals considerable variation in underlying age. Specialists studying meter, vocabulary and mythic content have argued that several Yashts, the Mihr Yasht and Aban Yasht prominent among them, carry material that predates the systematic form Zoroastrianism took under the Achaemenid and Sasanian empires, reshaped and given a Zoroastrian theological frame without fully erasing the older layer beneath it.
That unevenness makes the Yashts a frequent point of comparison for scholars studying the relationship between Zoroastrianism and the wider Indo-Iranian religious world it emerged from, since figures such as Mithra and Anahita have close counterparts and cognates elsewhere in Iranian and Indian tradition. The Yashts were transmitted orally for a very long period before being fixed in the Avestan alphabet devised under the Sasanians, and like the rest of the Avesta they were later accompanied by a Pahlavi Zand for priests who no longer read Avestan as a living language.
The Farvardin Yasht, honoring the Fravashis, the guardian spirits of the dead, is recited during the Zoroastrian new year period and remains among the most widely used of the collection in contemporary practice, which shows how individual Yashts keep a devotional life distinct from the set as a whole.