In this first book-length treatment of Descartes' important and influential natural philosophy, Daniel Garber is principally concerned with Descartes' accounts of matter and motion--the joint between Descartes' philosophical and scientific interests. These accounts constitute the point at which the metaphysical doctrines on God, the soul, and body, developed in writings like the Meditations, give rise to physical conclusions regarding atoms, vacua, and the laws that matter in motion must obey. Garber achieves a philosophically rigorous reading of Descartes that is sensitive to the historical and intellectual context in which he wrote. What emerges is a novel view of this familiar figure, at once unexpected and truer to the historical Descartes. The book begins with a discussion of Descartes' intellectual development and the larger project that frames his natural philosophy, the complete reform of all the sciences. After this introduction Garber thoroughly examines various aspects of Descartes' physics: the notion of body and its identification with extension; Descartes' rejection of the substantial forms of the scholastics; his relation to the atomistic tradition of atoms and the void; the concept of motion and the laws of motion, including Descartes' conservation principle, his laws of the persistence of motion, and his collision law; and the grounding of his laws in God.