This study was designed to determine whether mastery learning instruction at the secondary level is more effective than other instructional strategies. Seven teams of teachers in five school districts participated in the investigation. Subjects taught were middle school history and language arts, high school government, mathematics, history, and junior high science. After teams of teachers had been selected for the project, training in mastery learning was initiated. Staff development sessions included an overview session of the basic concepts of mastery learning and the specific strategies necessary for teaching for mastery. As mastery learning in project schools commenced and progressed, each teacher kept a daily or weekly log of reactions and comments about the project. At the end of the project period, each teacher summarized the process of using mastery learning in their subject. Although not all of the teachers completely endorsed mastery learning at the end of the project as a means to improve student achievement, they did agree that the training, and teaching using mastery learning had improved their planning and instruction. The teachers related that they had advanced in curriculum organization, test construction, and classroom management. Even those who had previously used individualized instruction felt the requirements of mastery learning had infused more specificity into their planning and teaching. (JD)