The Gathas, the oldest Zoroastrian scripture, are made up of 17 hymns. They are traditionally attributed to Zarathustra, or Zoroaster, and form the oldest poetic core of the Avesta, the sacred literature of Zoroastrianism.
The 17 hymns are arranged into five groups called Gathas: Ahunavaiti, Ushtavaiti, Spenta Mainyu, Vohu Khshathra, and Vahishto Ishti. Together, they contain 238 stanzas and are preserved within the Yasna, the principal liturgical collection of the Avesta. Their language is Old Avestan, closely related to the archaic Indo-Iranian language of early Vedic poetry.
The Gathas are not 17 separate books in the modern sense. Their hymns are identified by Yasna chapter numbers, mainly chapters 28–34, 43–46, 47–50, 51, and 53. The “21” sometimes associated with Zoroastrian scripture refers to the traditional division of the wider Avesta into 21 nasks, not to the number of Gathic hymns.