Outcomes of 28 peer-reviewed journal articles published between 1981 and 2007 were evaluated quantitatively to assess the effectiveness of classroom-wide interventions for the improvement of social skills. All interventions included in the study were implemented with intact classrooms that included both socially competent children and those with social skills difficulties. In general, the overall effect of school interventions on social behavior was positive but small (effect size = 0.15). Studies were further analyzed according to several variables of interest (e.g., grade of intervention, socioeconomic status, length of intervention, and so on). Several variables moderated the outcome. Particular attention was given to the finding that early interventions were more effective than interventions with older students. These results suggest that resources in classroom-based social skills interventions are best invested in younger students, particularly those in preschool and kindergarten. (Contains 2 tables.)