For the first time, the vowels of Avestan are studied comprehensively on the synchronic and diachronic level. All vowel changes which have occurred after the Proto-Iranian stage are discussed, and they are placed in a relative chronology. The phonological system of Avestan at various stages of its development is reconstructed, and the relationship between Old Avestan and Young Avestan is reviewed. Also, many philological details are discussed. This volume is of interest for Indo-Iranian philology, for Indo-European linguistics and for Iranian linguistics.