SummaryWhen asked “what is an emotion?” most people answer in one of three ways. One answer is to list the most salient attributes of emotions. The psychologist and philosopher William James, in an 1884 essay with the eponymous title of our question, causally linked two commonsense attributes. According to James, certain stimuli can trigger emotional bodily reactions, and our perception of those changes constitutes our conscious experience of emotions, feelings. We see a bear: our heart rate accelerates, our blood pressure shoots up, and many other bodily changes transpire. Our perception of those changes in our body constitutes our fear of the bear. More recent accounts propose neurobiological substrates involved in causing emotional reactions and perceiving the feelings, laying the foundation for conceiving of emotions as neural states. Modern emotion theories typically try to account for the observations that emotions are triggered by events of some significance or relevance to an organism, that they encompass a coordinated set of changes in brain and body, and that they appear adaptive in the sense that they are directed towards coping with whatever challenge was posed by the triggering event. Emotions also have an onset, a dynamic timecourse, and an offset or resolution; their phasic nature is one feature that distinguishes them from moods. Additional layers of complexity are added, especially in humans, through our capacity to control and regulate our emotions (at least to some extent), and to vicariously experience the emotions of other people through empathy, both of which are current major themes in emotion research.