What happens when someone acts? We suppose that we are often morally responsible for what we do, that our creations merit credit, and the unfolding of our relationships with others find their ultimate source within us - in the choices we have freely made. But how is such freedom of choice possible? What are the springs of free will? In this carefully-argued and provocative study, O'Connor systematically develops an account of human agency intended to shed light on these basic questions. Central to his account is the traditional concept of 'agent' of 'personal' causation, a concept that has been largely abandoned in contemporary discussions of free will. O'Connor critically assesses the previous account of this notion by Thomas Reid, Richard Taylor, and Roderick Chisholm, before reformulating it in relation to more general discussions of contemporary causation. He then provides an original account of how reasons can explain actions whose causes are their agents. He concludes by arguing at length that the freedom of will his account describes is consistent with an understanding of human beings as being fully rooted in the natural world. O'Connor also criticizes several alternative accounts of free will and offers fresh arguments bearing on several related topics, including: the incompatibility of freedom and determinism, Frankfurt's challenge to the link between freedom and moral responsibility, the nature of event causation, contrastive explanation, and the concept of emergence. This book will interest not only theorists of action and free will, but also philosophers of mind and general metaphysicians.