Occasionalism is commonly understood as a theory that ascribes all causal power to God on the one hand and treats cause-effect relations in nature as occasions indicating the manner of divine creation on the other. As the research on the historical background of Hume's critique of causality has been improved, western scholars have been increasingly attracted to occasionalism. Yet, current discussions of occasionalism have given rise to a number of paradoxes that make the doctrine appear to be inconsistent. In my opinion, these inconsistencies arise from misinformation and historical inaccuracies about occasionalism. Its Muslim roots are largely unknown and its early modem origins have not been studied sufficiently. As a result, ordinary reader may be familiar with certain ideas about occasionalism but they are often mixed with inaccurate characterizations of the theory. In my dissertation, I explore the roots of occasionalism and attempt to clarify it as a metaphysical theory. I show that a close reading of Islamic sources and critical comparison of early modern theories provide some answers to the many questions that have arisen in contemporary research on occasionalism. I bring a resolution to these problems by carefully studying the historical roots of occasionalism. I examine its historical development chronologically and comparatively in order to clarify the similarities and differences between different schools of thought. My dissertation falls mainly into two parts. The first part of the dissertation focuses on the Islamic roots of occasionalism. This theory was propounded first by the Muslim mutakallimun mainly within an atomistic cosmology. I look at the Mu`tazila, the Ash`arite and the Maturidite schools and al-Ghazali in their contributions to occasionalism. I argue that occasionalism lies in the foundation of the Sunni kalam. Al-Ghazali and the Sunni kalam , both the Ash`arite and Maturidite schools, uniformly embraced occasionalism though the descriptions of their cosmologies differ to a certain extent. The second part deals with the early modern roots of the theory by considering Descartes', Louis de La Forge's, Cordemoy's, and Malebranche's contributions to it. In this part, I argue that Descartes and La Forge settled the general framework in which Cordemoy and Malebranche developed an occasionalist metaphysics. Cordemoy presented his occasionalism in an atomistic cosmology although influenced from Cartesian philosophy, and Malebranche produced the most systematic version of occasionalism that is based on Cartesian metaphysics. By examining the historical roots of occasionalism, I show that it is a serious metaphysical theory fully capable of dealing with problems in traditional metaphysics. KEY WORDS: Occasionalism, causality, Islamic kalam , Cartesians, metaphysics