This is a book about the intellectual history of the later Sasanian Empire, with a particular focus on medicine, astral science (i.e., astronomy and astrology), and philosophy. It is also, necessarily, a book that deals extensively with the premodern Islamic world – not only because this is the context where most of the fundamental sources for Sasanian intellectual life take shape but also because the transition from Sasanian to Muslim rule in Iraq and Iran in the mid-seventh century CE did little to immediately and substantially impact this kind of scholarship. Particularly for the fields in focus here, a coherent intellectual-historical period can be said to extend from the sixth to the ninth century, as it is only around 800 that, owing to the raft of scholarly translations into Arabic produced under the early Abbasids, medicine, natural science, and philosophy in the erstwhile Sasanian heartland undergo a decisive shift: Greco-Roman scholarship achieves qualitatively new levels of centrality and availability, and Middle Persian and Syriac give way to Arabic as the dominant language of secular learning.