Literature on achievement effects of practical applications of group-based mastery learning in elementary and secondary schools over at least four weeks is examined, using the best-evidence synthesis review technique (a combination of meta-analysis and narrative reviews). There was essentially no evidence to support the effectiveness of group-based mastery learning on standardized achievement measures. Effects were generally positive, though moderate, on experimenter-made measures, with little evidence that they were maintained over time. These findings suggest the need for increased efforts to see how the compelling theories underlying mastery learning can achieve their potential. A 13-page list of references and three tables are presented. (SLD)