The importance of general celestial influences on the Earth in Aristotle’s cosmological model enabled the art of astrology to find a large degree of acceptance in intellectual circles by the mid-twelfth century, even if throughout the late Middle Ages it continued to be haunted by the debate about determinism. Astrology - or the study of the movements and relative positions of celestial bodies in order to make predictions about human personalities, dispositions, and public and personal events - included the belief that the planets could incline men to good and evil, and negatively influence the course of events. In this paper, Sophie Page examines how the question of whether or how demons could provoke, manipulate or make use of these celestial influences was of particular concern to three different types of medieval author: theologians explaining the structure and operations of the cosmos, authors of literary or popular scientific texts discussing the origins of evil in the world, and writers of texts on astrology and magic, whose main goal was to identify networks of power in the cosmos which could be manipulated by humans.