Behavior problems in children are an important social, educational, and health issue. The prevalence of these problems, their stability over time, their poor prognosis, and their costs to both individuals and the society, all point to the need for primary prevention and early effective interventions. A systematic review examined the effectiveness of group parent education programs that aimed to improve behavior problems in 3- to 10-year-old children. The phrase "parent education program" is used here to refer to group-based programs with a standardized format aimed at enhancing parenting skills. The term "behavior problems" is used to refer to children exhibiting externalizing problems such as temper tantrums, aggression, and noncompliance. It does not include children diagnosed as having attention-deficit hyperactivity disorder. This review focused explicitly on measures of child behavioral outcomes, which are only small, albeit important, outcomes of parent education programs. Reviews focusing on other clinically relevant outcomes are also needed, including parental well-being and attitudes towards parenting. Other reviews are also needed to collate evidence concerning the effectiveness of parent education programs with other age-groups, i.e., preschoolers and adolescents, and in improving other aspects of child well-being. The review included published studies only and as such may have been influenced by a "publication bias." Inclusion criteria comprised the use of a waiting list, a no-treatment or placebo control group, and at least one standardized measure assessing the child's behavior. Only studies published after 1970 that included at least one "group-based" parent education program were included. A total of 255 primary studies were identified, but only 16 of these and 2 follow-up studies met all of the specified inclusion criteria. Critical appraisal of these 16 studies revealed considerable heterogeneity in the interventions, the populations studied, and the outcome measures used. Nevertheless, these studies suggest that structured parent education programs can be effective in producing positive change in both parental perceptions and objective measures of children's behavior and that these changes are maintained over time. Because of the small number of controlled studies and their methodological variations, caution should be exercised before these findings are generalized broadly.