Research on effectiveness of school bullying interventions has lagged behind descriptive studies on this topic. The literature on bullying intervention research has only recently expanded to a point that allows for synthesis of findings across studies. The authors conducted a meta-analytic study of school bullying intervention research across the 25-year period from 1980 through 2004, identifying 16 studies that met our inclusion criteria. These studies included 15,386 K through 12 student participants from European nations and the United States. Applying standard meta-analysis techniques to obtain averaged effect size estimates across similar outcomes, the authors found that the intervention studies produced meaningful and clinically important positive effects for about one-third of the variables. The majority of outcomes evidenced no meaningful change, positive or negative. The authors conclude that school bullying interventions may produce modest positive outcomes; that they are more likely to influence knowledge, attitudes, and self-perceptions rather than actual bullying behaviors; and that the majority of outcome variables in intervention studies are not meaningfully impacted. (Contains 4 tables.)