Empathy shapes the landscape of our social lives. It motivates prosocial and caregiving behaviors, plays a role in inhibiting aggression, and facilitates cooperation between members of a similar social group. Thus, empathy is often conceived as a driving motivation of moral behavior and justice, and, as such, everyone would think that it should be cultivated. However, the relationships between empathy, morality, and justice are complex. We begin by explaining what the notion of empathy encompasses and then argue how sensitivity to others' needs has evolved in the context of parental care and group living. Next, we examine the multiple physiological, hormonal, and neural systems supporting empathy and its functions. One troubling but important corollary of this neuro-evolutionary model is that empathy produces social preferences that can conflict with fairness and justice. An understanding of the factors that mold our emotional response and caring motivation for others helps provide organizational principles and ultimately guides decision making in medical ethics.