Mastery learning can combine the strengths of direct instruction with the strengths of discovery learning. This paper describes how mastery learning is being used in Missouri schools to enhance the quality of instruction in science at all grade levels. In addition, the dramatic results in student science achievement that have occurred within the state since this program's inception are discussed. Missouri educators developed a list of science competencies all students should attain prior to completion of their secondary education. The Missouri Mastery Achievement Tests were developed using the principles that: (1) competencies should not be restricted to the easiest to assess; and (2) competencies and skills should be mainly higher order outcomes. (PR)