iolence is clearly contagious. If youth are exposed to violence, their own risk of behaving violently increases. The violence can be real-world proximal violence or unreal violence in the mass media. Furthermore, the effects of observing violence are more than transitory. I have argued that this happens because various social cognitions and emotional reactions are acquired through observational learning and subsequently promote violent and aggressive behavior. In this presentation I present data from two types longitudinal studies. One type examines the role of observations of proximal real-world violence on subsequent aggressive behavior. The other examines the role of exposure to media violence or playing violent video games on subsequent aggressive behavior. In both studies we find that exposure over time leads to a subsequent increased risk of behaving aggressively. In both types of studies I find that the contagion effect appears to be mediated by increased fantasizing about aggression and by the acquisition of normative beliefs more accepting of aggression.