Across the educational systems of the world, few issues have received more attention in recent years than the problem of ensuring that elementary- and secondary-school classrooms are all staffed with adequately qualified teachers (Mullis et al., 2000; OECD, 1994, 2005; Wang et al., 2003). Even in nations where students routinely score high on international exams, the issue of teacher quality is the subject of much concern. This is not surprising. Elementary and secondary schooling is mandatory in almost all nations and children are legally placed in the care of teachers for a significant portion of their lives. It is widely believed that the quality of teachers and teaching are among the most important factors shaping the learning and growth of students. Moreover, this impact goes beyond student academic achievement. Across the world, observers routinely tie the performance of teachers to numerous, larger societal goals and problems - economic competitiveness and productivity, juvenile delinquency, moral and civic culture, and so on.