This book presents an account of normative practical reasons and the way in which they contribute to the rationality of action. Rather than simply 'counting in favour of' actions, normative reasons play two logically distinct roles: requiring action and justifying action. The distinction between these two roles explains why some reasons do not seem relevant to the rational status of an action unless the agent cares about them, while other reasons retain all their force regardless of the agent's attitude. It also explains why the class of rationally permissible action is wide enough to contain not only all morally required action, but also much selfish and immoral action. The book will appeal to a range of readers interested in practical reason in particular, and moral theory more generally.