Educational institutions have at least two major functions: education and certification of competency. This paper examines the educational strengths and limitations of the mastery learning instruction model with respect to fulfilling these functions. The components of the mastery model are contrasted with components of other instructional models, and their relative advantages and disadvantages discussed in the nine sections of the paper. Components of the mastery model are identified as: (1) formal specification of a comprehensive set of cognitive objectives; (2) instruction; (3) frequent formative diagnostic evaluation; (4) corrective or remedial instruction measures to remedy learning deficiencies identified in formative evaluation; and (5) criterion referenced summative evaluation. The advantage of mastery instruction is primarily its effectiveness for teaching basic skills and knowledge to slow learners and students who have not learned how to learn. Consequently, its optimal usefulness is in the elementary grades, especially primary grades. The model reduces competition among students and reduces student failure and frustration. It is also effective with disadvantaged students and slow learners at all educational levels. The model does not do well when implemented in traditionally organized schools with time-fixed instructional units. It does not maximize learning for all students. However, the inclusion of an idea of mastery speed along with mastery certification would make mastery grades useful for educational and vocational decision-making. (DMT)