The Avesta is a collection of liturgical texts considered as their sacred book by the Zoroastrian community. It contains the recitatives of the Zoroastrian liturgies still celebrated in the 17th century, some of them even celebrated until today. The texts integrated in these ceremonies were composed in different places and at different times, and transmitted orally for centuries. The exact date of the fixation of the ceremonies in the shape in which they are presented in the manuscripts and the creation of the different manuscripts is unknown. But today it is proven that even after the creation of the first manuscripts, the transmission of these liturgical texts was the result of a complicated process in which not only the process of copying manuscripts but also the ritual practice and the ritual teaching were involved. The only deep analysis of the written transmission of the Avesta was made by K. F. Geldner as Prolegomena to his edition of the Avesta. Since then, many new manuscripts have appeared. In this volume contributions by the main experts in the transmission of the Avesta are gathered: the oral transmission, the fixation of the different collections, the first writing down, and the manuscripts. Special interest is devoted to the manuscripts. Some contributions of the volume were presented at the correspondent colloquium held in Salamanca, September 2009; others were added in order to make of the volume a comprehensive work on the different aspects of the Avestan transmission.