Theinitial aim of the Chair of Indo-Iranian Languages and Religions, which Jean Kellens held from 1993 to 2014, was to compare the most ancient texts of India (the Veda) with those of Iran (theAvesta), whose language and religious conceptions had remained very similar. At the same time, the exploration of the texts had to be combined with the concern to produce a new Avestic dictionary, which seemed a necessity at the time. Very quickly, the examination carried out in this perspective revealed the real structure of theAvesta : not, as had been thought until then, the haphazard anthology of the debris of a vast lost collection, but the concerted assembly of texts corresponding to various complex and coherent sacrificial ceremonies. From then on, the real and complex structure of theAvesta became the main concern.