My dissertation addresses a foundational problem in the philosophy of action, that of explaining the distinction between actions and mere events. Actions, I argue, have a uniquely active component that distinguishes them from mere events and which can be explained in terms of effort. Effort has several features: it is attributed directly to agents; it is a causal power that each agent alone possesses and employs; it enables agents causally to activate, sustain, and control their capacities during the performance of an action; and its presence comes in varying degrees of strength. After defending an effort-based account of action and criticizing what is known as the standard story of action, I apply my account to situations in which an agent...