This paper reviews the literature on mastery learning theory and reports its successful application since 1987 with mainstreamed special education students at one elementary school in Missouri. The methodology, originally developed by Benjamin Bloom, involves formative evaluation after initial instruction followed by either corrective activities and reevaluation or by enrichment activities. Use of mastery learning is reported to result in substantially increased student achievement for both regular and special education students. At the Thorpe-Gordon elementary school in Jefferson City, Missouri, the techniques of mastery learning are being used in regular classes in which 40 mildly disabled and at risk students from the fourth, fifth, and sixth grades are mainstreamed. The regular class teacher provides the initial instruction and administers the first formative assessment. Corrective instruction is then provided within the classroom to students who have not attained mastery while the regular teacher provides enrichment activities to the remaining students. Standardized test results show that only 10%, compared to 40% previously, of students achieve in the bottom two quintiles with 50-75% achieving in the highest quintile. Grades of the mainstreamed students and comments of teachers also support the program's success. Includes 19 references. (DB)