This meta-analysis synthesized bullying prevention programs' effectiveness at increasing bystander intervention in bullying situations. Evidence from 12 school-based programs, involving 12,874 students, indicated that overall the programs were successful (Hedges's g = 0.20, 95% confidence interval [CI] = 0.11 to 0.29, p = 0.001), with larger effects for high school (HS) samples compared to kindergarten through eighth-grade (K-8) student samples (HS effect size [ES] = 0.43, K-8 ES = 0.14; p less than 0.05). A secondary synthesis from eight of the studies that reported empathy for the victim revealed treatment effectiveness that was positive but not significantly different from zero (g = 0.05, 95% CI = -0.07 to 0.17, p = 0.45). Nevertheless, this meta-analysis indicated that programs increased bystander intervention both on a practical and statistically significant level. These results suggest that researchers and school administrators should consider implementing programs that focus on bystander intervention behavior supplementary to bullying prevention programs. (Contains 4 tables and 1 figure.)