Abstract Prosocial behavior covers the broad range of actions intended to benefit one or more people other than oneself—actions such as helping, comforting, sharing, and cooperation. Altruism is motivation to increase another person's welfare; it is contrasted to egoism, the motivation to increase one's own welfare. There is no one-to-one correspondence between prosocial behavior and altruism. Prosocial behavior need not be motivated by altruism; altruistic motivation need not produce prosocial behavior. Over the past 30 years, the practical concern to promote prosocial behavior has led to both a variance-accounted-for empirical approach, which focuses on identifying situational and dispositional determinants of helping, and the application of existing psychological theories. Theories invoked to explain prosocial behavior include social learning, tension reduction, norm, exchange or equity, attribution, esteem-enhancement, and moral reasoning theories. In addition, new theoretical perspectives have been developed by researchers focused on anomalous aspects of why people do—and don't—act prosocially. Their research has raised the possibility of a multiplicity of social motives—altruism, collectivism, and principlism, as well as egoism. It has also raised questions—as yet unanswered—about how these motives might be most effectively orchestrated to increase prosocial behavior.