The purpose of this study was to determine the extent to which mastery testing contributes to instructional effectiveness and to determine the size of its effects in different outcome areas, such as aptitude and instructional time. A meta-analytic methodology was used to integrate findings from experimental studies. Twenty studies examined college-level effects, and investigated pre-college effects. Each study compared students taught with a mastery requirement on tests to students taught in the same way but without a requirement of test mastery. The results showed that testing for mastery had effects on student achievement at both college and pre-college levels. The effects of mastery testing were strong on low aptitude students. A mastery requirement had the effect of diminishing individual differences in student achievement in classes and of reducing the correlation between aptitude and achievement. Results also showed that mastery testing increased the amount of instructional time. (JAZ)