This study offers a new account of the development of Cartesian Occasionalism. The doctrine of Occasionalism - most famously advocated by Nicolas Malebranche - states that God alone is the cause of every event, and created substances are merely "occasional causes." In the years following René Descartes' death in 1650, several of his followers -- including Arnold Geulincx, Gerauld de Cordemoy and Louis de la Forge - argued for some version of this thesis. My study builds on recent scholarship about these first Cartesian Occasionalists, the motives that led them and Malebranche to adopt Occasionalism, and the connections between Occasionalism and the views of Descartes. I analyze the doctrine of Occasionalism, and examine its relationship to Descartes' philosophy. I argue that Descartes' views in physics and metaphysics are consistent with the claim that corporeal substances have intrinsic causal powers, and reply to arguments by Gary Hatfield, Janet Broughton and Daniel Garber that purport to show otherwise. I examine how Occasionalism relates to several proposed problems with Descartes' claim that the human mind causally interacts with the human body, and argue that Occasionalism is not the "natural" solution to these problems. I conclude by canvassing the arguments of the earliest Cartesian Occasionalists. I argue Cartesian Occasionalism was neither an ad hoc solution to the mindbody problem, nor a logical consequence of Descartes' own views, but rather an attempt to extend and better systematize Cartesian philosophy.