Research on the prevention and treatment of aggression is vitally important because the emergence of early onset ODD/CD in preschool children is stable over time and appears to be the single most important behavioural risk factor predictive of antisocial behaviour for boys and girls in adolescence. In particular, physically aggressive behaviour in children as young as age three has repeatedly been found to predict the development of violent juvenile delinquency and drug abuse in adolescence, as well as depression and school dropout rates. There is some suggestion that, in the absence of intervention, early starter aggressive tendencies in children may crystallize around age eight. At this point in life, learning and behavioural problems may become less amenable to intervention and more likely to develop into a chronic disorder. Since treatment of aggression becomes increasingly difficult and more costly as children grow older, it seems both pragmatic and cost effective to offer treatment and prevention efforts during the toddler and preschool years. Unfortunately, recent projections suggest that less than 10% of school-aged children (and even fewer preschool-aged children) who need services for aggressive behaviour actually receive them; and less than half of this group receive empirically validated interventions.