Although much recent attention has focused on gaps in the achievement of different groups of students, the problem has been with us for decades. This paper presents the problem as one of reducing variation in students' achievement, and reviews the work of renowned educator Benjamin Bloom on this problem. Bloom argued that to reduce variation in students' achievement and to have all students learn well, we must increase variation in instructional approaches and learning time. The key element in this effort is well constructed, formative classroom assessments. Bloom outlined a specific strategy for using formative classroom assessments to guide teachers in differentiating their instruction and labeled it "mastery learning." This paper describes Bloom's work, presents the essential elements of mastery learning, explains common misinterpretations, and describes the results of research on its effects. (Contains 4 figures.)