 As others before him, Avicenna understands natural philosophy as the study of body insofar as it is subject to motion. Thus the present entry, after an overview of medieval physics generally, turns to Avicenna’s account of the nature of body followed by his account of motion. Central to an account of bodies is whether they are continuous or atomic, and so one must consider Avicenna’s critique of atomism and his defense and analysis of continuous magnitudes. The discussion of motion follows in two steps: one, his account of motion and, two, his account of the conditions for motion. The section on motion concentrates on Avicenna’s unique understanding of the Aristotelian definition of motion as the first actuality of potential as potential, while the conditions for motion involve his understanding of time and place. 