The problem of achievement gaps among different subgroups of students has been evident in education for many years. This manuscript revisits the work of renowned educator Benjamin S. Bloom, who saw reducing gaps in the achievement of various groups of students as a simple problem of reducing variation in student learning outcomes. Bloom observed that teaching all students in the same way and giving all the same time to learn--that is, providing little variation in the instruction--typically results in great variation in student learning. Students for whom the instructional methods and amount of time are appropriate learn well, and those for whom the methods and time are less appropriate learn less well. Bloom believed that all students could be helped to reach a high criterion of learning if both the instructional methods and time were varied to better match students' individual learning needs. In other words, to reduce variation in the achievement of diverse groups of students and have all students learn well, Bloom argued that educators and teachers must increase variation in instructional approaches and learning time. Bloom labeled the strategy to accomplish this instructional variation and differentiation mastery learning. Research evidence shows that the positive effects of mastery learning are not limited to cognitive or achievement outcomes. The process also yields improvements in students' confidence in learning situations, school attendance rates, involvement in class sessions, attitudes toward learning, and a variety of other affective measures. (Contains 4 figures.)