Few educational issues have received more attention in recent times than the problem of ensuring that our nation's elementary and secondary classrooms are all staffed with quality teachers. Concern with the quality of teachers is neither unique nor surprising. Elementary and secondary schooling are mandatory in the United States, and children are legally placed into the custody of teachers for a significant portion of their lives. The quality of teachers and teaching are undoubtedly among the most important factors shaping the learning and growth of students. Moreover, the largest single component of the cost of education is teacher compensation.