 No aspect of our mental life is more important to the quality and meaning of our existence than the emotions. They are what make life worth living and sometimes worth ending. So it is not surprising that most of the great classical philosophers had recognizable theories of emotions.  These theories typically conceived of emotions as a subject’s phenomenologically salient responses to significant events and as capable of triggering distinctive bodily changes and behaviors. But it is surprising that throughout much of the twentieth-century, scientists and philosophers of mind tended to neglect the emotions—in part because of behaviorism’s allergy to inner mental states and in part because the variety of phenomena covered by the word “emotion” discourages tidy theorizing. In recent decades, however, emotions have once again become the focus of vigorous interest in philosophy and affective science. Our objective in this entry is to account for these developments, focusing primarily on the descriptive question of what the emotions are, but tackling also the normative question of whether emotions are rational. In view of the proliferation of exchanges between researchers of different stripes, it is no longer useful to speak of the philosophy of emotion in isolation from the approaches of other disciplines, particularly psychology, neuroscience, and evolutionary biology. This is why we have made an effort to pay significant attention to scientific developments, as we are convinced that cross-disciplinary fertilization is our best chance for making progress in emotion theory.