This is the first modern comprehensive account of the syntax of Old Avestan, the earliest known form of Iranian language, attested in the Gathas of Zarathushtra and the Liturgy in Seven Chapters. It is based on the most up-to-date understanding of the texts, while following traditional principles of grammatical analysis. There are also substantial sections on word order, stylistics, and figures of speech. Translations are provided for almost all passages quoted. The work will be welcomed by Iranianists as well as by historical linguists with wider Indo-European interests.