Although much has been written recently about gaps in the achievement of different groups of students, the problem has been with us for many years. This manuscript presents a historical perspective of the problem, viewing it as one of reducing variation in students' achievement. Specifically, it reviews the work of renowned educator Benjamin S. Bloom, who argued that to reduce variation in students' achievement and have all students learn well, we must increase variation in instructional approaches and learning time. Bloom also outlined a specific strategy to accomplish this instructional differentiation, labeling it "mastery learning." Bloom's work is described, along with common misinterpretations of his ideas and the results of research on the effectiveness of their implementation.