In very general terms, an agent is a being with the capacity to act, and 'agency' denotes the exercise or manifestation of this capacity. The philosophy of action provides us with a standard conception and a standard theory of action. The former construes action in terms of intentionality, the latter explains the intentionality of action in terms of causation by the agent’s mental states and events. From this, we obtain a standard conception and a standard theory of agency. There are (...) alternative conceptions of agency, and it has been argued that the standard theory fails to capture agency. Further, it seems that genuine agency can be exhibited by beings that are not capable of intentional action, and it has been argued that agency can and should be explained without reference to causally efficacious mental states and events. Debates about the nature of agency have flourished over the past few decades in philosophy and in other areas of research. 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. (shrink)