Debates about the nature of agency have flourished over the past few decades in philosophy and in other areas of research (including psychology, cognitive neuroscience, social science, and anthropology). In philosophy, the nature of agency is an important issue in the philosophy of mind, the philosophy of psychology, the debates on free will and moral responsibility, in ethics, meta-ethics, and in the debates on the nature of reasons and practical rationality. For the most part, this entry focuses on conceptual and metaphysical questions concerning the nature of agency. In the final sections, it provides an overview of empirically informed accounts of the sense of agency and of various empirical challenges to the commonsense assumption that our reasons and our conscious intentions make a real difference to how we act.