This chapter looks at theories of world soul in the medieval period, considering texts from the Islamic world and Latin Christendom. The central theme is the comparison between the cosmos and an individual human, who is conceived as a so-called microcosm. By this logic, since the human has a soul, so must the cosmos. Plato’s Timaeus is shown to be a key source for both cultures, including in Christian authors who detected a reference to the Holy Spirit behind Plato’s notion of World Soul. Figures in focus include al-Razi, the Brethren of Purity, the School of Chartres, Peter Abelard, and Hildegard of Bingen.