In my research I focus on theories of abilities in medieval and early modern philosophy. I am mostly interested in two sets of problems. First, I want to examine the metaphysical status that was ascribed to abilities in Aristotelian and non-Aristotelian theories. Were abilities considered to be fundamental entities that cannot be dispensed with in an analysis of human beings? Or were they reduced to other entities? If so, what was the driving force behind the reductionist program? Second, I intend to look at the explanatory role abilities played in theories of action. Many medieval and early modern authors claimed that there could be no action without the use of intellect and will. But what are these rational abilities? How do they relate to non-rational abilities? And why does the use of intellect and will mark the difference between action and mere behavior? In my analysis of these problems, I will look at a number of authors, ranging from thirteenth-century Aristotelians to seventeenth-century Cartesians and Leibniz.