Aggressive impulses arise from many factors, but they are usually held in check by social norms for self-control. Thus, the proximal cause of aggression is often failure of self-restraint. In five studies, depleted capacity for self-regulation (caused by prior, even irrelevant acts of self-regulation) increased aggressive responding, especially after an insulting provocation. When participants were insulted and their self-regulatory strength was depleted (i.e., after completing previous tasks that required self-regulation), participants were more likely to aggress. When the urge to aggress was relatively weaker (i.e., when participants were not insulted), self-regulatory depletion did not increase aggressive behavior. This effect was moderated by trait self-control: Participants low in trait self-control were particularly likely to express intentions of behaving aggressively in response to provocation, whereas participants high in trait self-control did not express intentions of responding aggressively. Laboratory, autobiographical memory, and hypothetical responses confirmed the pattern.