Objective: Researchers have identified several family-based treatments that hold considerable promise in reducing serious juvenile offending; however, these treatments remain underutilized by youth service systems. In the present study, we used meta-analysis to summarize the findings of research on family-based treatments for serious juvenile offenders. Method: We conducted a multilevel meta-analysis that modeled dependencies between multiple effect sizes from the same study. The meta-analysis synthesized 324 effect sizes from 28 studies that met inclusion criteria. Potential moderators (e.g., characteristics of samples, treatments, methods, and measures) were entered as fixed effects in the meta-analytic model. Results: Across studies, family-based treatments produced modest, yet long-lasting, treatment effects (mean d = 0.25 for antisocial behavior, 0.24 overall) relative to comparison conditions. Furthermore, certain characteristics moderated the magnitude of treatment effects; for example, measures of substance use showed the largest effects and measures of peer relationships showed the smallest effects. Conclusions: Policymakers, administrators, and treatment providers may find it useful to consider the effects of family-based treatments for serious juvenile offenders in their selection of treatments for this population. In addition, investigators who seek to develop and study such treatments may wish to consider the current findings in their future research efforts.