The book deals with some major aspects of Zoroastrianism in Iran during the Sasanian period, including the important distinctions between the spritual and the material modes of existence, the idea that Ahreman, the Evil Spirit, does not belong in the material world, and the widely current myth of Zurvan. The volume also deals with the impact of some Zoroastrian themes on Islam, such as the twin relationship between state and religion, throwing light on both the Iranian themes and their borrowing into Islamic literature.