There is continuing debate on the extent of the effects of media violence on children and young people, and how to investigate these effects. The aim of this review is to consider the research evidence from a public-health perspective. A search of published work revealed five meta-analytic reviews and one quasi-systematic review, all of which were from North America. There is consistent evidence that violent imagery in television, film and video, and computer games has substantial short-term effects on arousal, thoughts, and emotions, increasing the likelihood of aggressive or fearful behaviour in younger children, especially in boys. The evidence becomes inconsistent when considering older children and teenagers, and long-term outcomes for all ages. The multifactorial nature of aggression is emphasised, together with the methodological difficulties of showing causation. Nevertheless, a small but significant association is shown in the research, with an effect size that has a substantial effect on public health. By Contrast, only weak evidence from correlation studies links media violence directly to crime.