Children witness violence at home, at school, in their neighborhood, and in the media. Children may also experience violence, as a victim, at home, at school, and in their neighborhood. A longitudinal study tested whether children who are exposed to a heavy dose of violence come to regard it as normal behavior and subsequently behave more aggressively themselves. Participants were 777 children (8 to 12 years old) who completed questionnaires twice (6 months apart) about exposure to violence (witnessed and experienced), their own aggression, the aggression of peers, and normative beliefs about aggression. The results showed that witnessing violence predicted increases in aggression 6 months later through changes in normative beliefs. Likewise, experiencing aggression as a victim predicted increases in aggression 6 months later through changes in normative beliefs. These findings show that when children think violence is commonplace in many contexts, they are more likely to aggress against others. © The Author(s) 2011.