Violence in screen entertainment media (television, film, video games)—defined as depictions of characters (or players) trying to harm other characters (or players)— is ubiquitous. The Workgroup on Media Violence and Violent Video Games reviewed numerous meta-analyses and other relevant research extending 60+ years into the past, with an emphasis on more recent work on violent video games. The workgroup sought compelling evidence of both short-term and long-term effects. Questions included whether media violence increases, decreases or has no effect on: a) aggressive behavior; (b) aggressive thoughts, cognitions, beliefs…; (c) aggressive affect; (d) prosocial behavior; and (e) empathy for/desensitization to violence to others. Furthermore, the workgroup explains how and why effects occur in terms of modern social-cognitive theories of social behavior. The Workgroup report also notes major needs for additional research.