Objective: The authors investigated the unique contribution motivational interviewing (MI) has on counseling outcomes and how MI compares with other interventions. Method: A total of 119 studies were subjected to a meta-analysis. Targeted outcomes included substance use (tobacco, alcohol, drugs, marijuana), health-related behaviors (diet, exercise, safe sex), gambling, and engagement in treatment variables. Results: Judged against weak comparison groups, MI produced statistically significant, durable results in the small effect range (average g = 0.28). Judged against specific treatments, MI produced nonsignificant results (average g = 0.09). MI was robust across many moderators, although feedback (Motivational Enhancement Therapy [MET]), delivery time, manualization, delivery mode (group vs. individual), and ethnicity moderated outcomes. Conclusions: MI contributes to counseling efforts, and results are influenced by participant and delivery factors. (Contains 6 tables.)