Since the publication of Anthony Kenny’s Action, Emotion and Will, there has been a consensus that emotions involve representing their objects as mattering to the subject in a certain way. All major contemporary theories aim to accommodate this characteristic, and those that clearly cannot have generally been abandoned. This essay is an investigation of a related but distinct characteristic of emotion, one that has often been mentioned but that is only beginning to be systematically investigated. This is that emotions are candidates for rationality: it is unreasonable to be angry with someone who is blameless, to envy someone who is wretched, or to be outraged about something that is unproblematic. In this essay, I offer an account of exactly what this rationality consists in, I argue that existing theories of emotion cannot wholly accommodate it, and I develop a theory that can. Chapter One is a critique of the currently ascendant perception theories, and Chapter Two is a critique of the formerly ascendant belief theories. Chapter Three outlines a positive account of emotions, which I characterise as a form of ‘non-doxastic cognitivism’. Chapters Four and Five look at some more specific features of emotion, namely their hedonic character and their potential for sentimentality.