Why do some leaders use their position to amass personal prestige and resources, and others to benefit the team, the organization, or society? This article synthesizes new, cross-disciplinary research showing that self-serving leader behavior is predictable based on the function and nature of power—an essential component of leadership. First, because power increases goal-oriented behavior, it amplifies the tendency of self-focused goals to yield self-interested behavior. Self-focused goals may arise from a variety of sources; evidence is reviewed for the role of traits (e.g., low agreeableness), values (e.g., self-enhancement), self-construal (e.g., independence), and motivation (e.g., personalized power motivation). Second, because power is generally desirable, leaders whose power is threatened (e.g., self-doubts, positional instability) will turn their focus to maintaining that power—even at others’ expense. These ideas have important implications for research and for organizational efforts to develop leaders who will improve others’ outcomes rather than merely benefit themselves.