A review of 61 interventions to reduce the rate of diagnostic testing revealed that many approaches to reducing test use yielded favorable results; the mean reduction observed was 22.0%. The heterogeneity of the interventions, target populations, and type of testing to be reduced did not permit carrying out a meta-analysis. However, this heterogeneity did permit the study of the relationship between indexes of program quality and effectiveness. Prior to implementation, few program developers measured the levels of excessive testing or developed conceptual foundations for the interventions. Interventions that were based on analyses of need and concerns about negative side effects tended to be more effective. It is suggested that focusing interventions on processes thought to foster the development of expert decision-making skills would improve the likelihood that interventions would show long-term effectiveness.