The text Wīdēwdād - “Law Serving to Keep the Demons Away” - is one of the longest and most important sources for the study of the Zoroastrianism of the ancient Iranian and the Middle Iranian periods. The ancient Iranian text, written in Avestan, was in the Sassanid era (3rd-7th centuries) translated into Middle Persian (Pahlavi) and provided with glosses and extensive commentaries. The Pahlavi version, called zand, is of particular interest for two reasons: firstly, it is the oldest Middle Persian translation of an Avestan text, and thus of major importance for the linguistic reconstruction of Middle Persian; secondly, the annotations approach complex theological, ritual, and legal questions that examine numerous insufficiently studied areas of the Sassanid society. Despite its outstanding importance, this primary source has, due to the high degree of difficulty of the subject matter, until recently attracted hardly any attention. Miguel Ángel Andrés-Toledo’s book, based upon a careful collation of all 44 still existing manuscripts, is the first critical edition of the Avestan and the Pahlavi text of the Wīdēwdād.