A meta-analysis of recent quantitative studies that examine the effects of cooperative learning (CL) on achievement outcomes in chemistry is presented. Findings from 25 chemical education studies involving 3985 participants (N[subscript treatment] = 1,845; N[subscript control] = 2,140) and published since 2001 show positive association between chemistry achievement and CL use (mean effect size = 0.68, Z = 5.04, p < 0.0001). Practically, an effect size of 0.68 suggests median student performance in a CL group would be at the 75th percentile when compared to that of a student in a traditional group performing at the 50th percentile. However, heterogeneity analysis indicated significant variability among the estimated effect sizes (Q = 248.51, df = 24, p < 0.0001), indicating other factors moderate the effects of CL. Analysis of class size, geographical location, and grade level as possible moderators suggested geographical location accounts for 27% of estimated 40% total heterogeneity (t[superscript 2] = 0.40, se = 0.13) and had significant influence (p = 0.003) on effect size estimates. For example, when only US-based studies are considered, the effect size decreases to 0.38, still an improvement of about 15 percentile points but nevertheless much smaller than the omnibus effect size. The findings of these analyses as well the study's implications are discussed.