Effective curriculum is based on a mastery learning model. According to Carroll's mastery learning model, learning rate is partially a function of student "perseverance" or time on task, which is in turn determined by a learner's entering repertoire and the quality of instruction. Although the mastery-learning model looks simple, its execution is enormously difficult. Difficulties include designing scope and sequence of curriculum, varying instructional time to fit different learning rates, varying content and level, and sustaining administrator commitment. Since implementing mastery-based curriculum is so difficult, it is often distorted into more traditional curriculum. This means that mastery-based, assessment-driven curriculum is often distorted into time-based, assignment-driven curriculum. At the curricular level, this distortion is evidenced by such things as low motivation, tests that do not truly measure mastery, and traditional grading. At the instructional level, the seven criteria for mastery learning are often distorted. These criteria include precise definition of instructional objectives, direct instruction, immediate feedback, appropriate level of instruction, modular learning, stimulus control, and critical response. Although research shows mastery learning to have superior results, it will not be successful in schools unless its distortions are eliminated. (JM)