Media violence and its effects on aggression is one of the most heavily investigated topics in the field of communication. Every time a child or teenager committed an act of violence in recent years, the debate about the effects of media violence on aggression flared up again. Can children and teens indeed become aggressive, or even criminal, from seeing violence on television, in movies, or in games? And if so, are some children and teens particularly vulnerable to media violence effects? This chapter reviews the latest findings on the effects of media violence on aggression and criminal behavior. It first discusses key studies that investigated the effects of media violence on aggression. It then discusses the most important theories of why and how media violence may stimulate aggression. Finally, it reflects on how and why some children may be more—and others less—susceptible to media violence effects.