Benjamin Bloom observed in his research on instructional practices and individual differences in learning that many teachers do not include adequate variation in instructional practices as they teach all students in the same manner and provide the same amount of time for everyone. According to Bloom, this approach caters to the set of students (about one-third) for which the instructional strategies and amount of time are suitable. This set of students perform well. Teachers also expect another one-third of the students to learn what is being taught but definitely not enough for them to be graded as good students. Bloom states that the last one-third are expected to fail or just manage to pass. In this way, teachers perpetuate the normal curve for grading students as a reference point in the school system such that failures are determined by the rank order of students instead of their lack of understanding of the main ideas of a course. Bloom discovered that a small variation in teaching strategies resulted in huge variation in learning outcomes. He was of the view that since students differ in how they learn (learning style) and what they can learn (aptitude), teachers need to provide for these individual differences through diversification and differentiation of instruction. The entire idea is encapsulated in mastery learning which is to the effect that if students’ ability for a course of study is distributably-normal, but we give each student sufficient time, help, and encouragement, then the distribution of grades will not conform to the normal curve because the correlation between aptitude and achievement will tend towards zero. The focus of this chapter is on mastery learning and how it may be applied in science teaching.