Moral exclusion occurs when individuals or groups are perceived as outside the boundary in which moral values, rules, and considerations of fairness apply. Those who are morally excluded are perceived as nonentities, expendable, or undeserving. Consequently, harming or exploiting them appears to be appropriate, acceptable, or just. This broad definition encompasses both severe and mild forms of moral exclusion, from genocide to discrimination. The paper discusses the antecedents and symptoms of moral exclusion, and the interaction between the psychological and social factors that foster its development. Empirical research on moral exclusion is needed to pinpoint its causes, to predict its progression, and to effect change in social issues that involve the removal of victims from our moral communities. The last section of the paper introduces the articles that follow.