This article discusses the emotional reactions most commonly associated with empathy and their relation to prosocial or altruistic action, aggression inhibition, and understanding others. In What is Empathy?, I characterize the distinct emotional reactions most commonly associated with empathy: empathy, sympathy, personal distress, and emotional contagion. In Measures of Empathy, I discuss the most common measures of dispositional and situational empathy. In Empathy, Prosocial Action, and Altruism, I consider the evidence that empathy, sympathy, and personal distress induce prosocial motivation. I conclude that sympathy is most strongly associated with prosocial, even altruistic, motivation. In Empathy and Aggression Inhibition, I examine the evidence that empathy inhibits aggression. The evidence is inconclusive. In Empathy and Mindreading, I briefly discuss empathy and mindreading, with an eye toward recent evidence concerning mirror neurons. I conclude by linking our current understanding of empathy to the philosophical tradition, and by offering some speculative remarks. WIREs Cogn Sci 2012, 3:253263. doi: 10.1002/wcs.1165