Aggression unopposed becomes a contagious disease.-Jimmy CarterFrom the scuffs and skirmishes that can be found on nearly any school playground to the cruelty and anonymity of cyberbullying, aggression is one aspect of child development that cannot be ignored. The primary aim of this chapter is to examine the development of aggression in childhood. The chapter fi rst begins with an overview of defi nitions. There are many different forms of aggression, and much controversy surrounding what terms to use. Therefore, an understanding of these issues is important before examining the actual development of aggressive behavior. Next, we will provide a developmental view of aggressive behavior and examine the stability of aggression over time. This is followed by an examination of sex differences in aggression, which shows that the stereotype of the " aggressive boy " and the " good girl " is incorrect. We then turn to the consequences of aggressive behavior to demonstrate that aggression hurts not only the victims of the behavior but also the aggressors themselves. Many different factors contribute to the development of aggressive behavior, much of which are beyond the scope of this chapter. However, the next section of the chapter will examine several areas, namely, biology, parenting and family, the peer context, and the media, all of which have been shown to infl uence aggressive behavior in children. Understanding the causes and consequences of aggression is not enough; therefore, we will next examine several interventions that have been found to decrease childhood aggression. Finally, we will suggest future directions that may advance our understanding of aggressive behavior in children.