Aggression brings tremendous costs to individuals, relationships, and society. Yet, people behave aggressively toward strangers and close others at alarmingly high rates. The current article seeks to unlock part of the mystery of why people behave aggressively. We review evidence that self-control failure plays an integral role in many acts of aggression and violence. We begin by reviewing theoretical models that emphasize the importance of self-control processes in understanding aggressive and other criminal behaviors. We also discuss how a theoretical model that originally neglected self-control processes can be extended to incorporate self-control theorizing. We then discuss recent empirical evidence (a) showing that self-control failure is a crucial predictor of aggression toward strangers and romantic partners and (b) identifying the neural processes relevant to the self-control of aggression. Finally, we review evidence that self-control processes can also explain why people engage in displaced aggression toward bystanders. By appreciating the importance of self-control processes, researchers and laypersons can gain a better understanding of why people behave aggressively - and how aggression can be prevented.