The concept of "teacher quality" has undergone a profound transformation in the last decade. Through the late 1990s, most policymakers assumed that educator effectiveness was immeasurable and that our only hopes to increase it were tied to classroom experience and academic credentials. Yet since 2001, through a series of notable research findings, changing political pressures, and landmark policy changes, we have come to view teacher quality as independent of licensure and individually measurable. We now approach evaluating the quality of our teachers by measuring their ongoing performance in the classroom. The key markers in the transformation: (1) Declining trust in teacher education: Data from the Higher Education Act of 1998 showed that teacher education programs graduated students of widely variable ability; (2) Ineffective credential requirements: Despite federal requirements for high-quality teachers, by 2006, students in high-poverty schools were still more likely to have lower-quality teachers than their peers in other schools; (3) Bipartisan agreement: By 2009, major federal politicians in both political parties, including Democratic politicians heavily supported by unions, agreed that low teacher quality was a barrier to high-quality education; and (4) New era of measurement: No Child Left Behind created a treasure trove of individual student data that, though imperfect in many ways, led to new opportunities for research to link teachers and students. No consensus has emerged about how to improve teacher quality, but policymakers continue to experiment with teacher preparation programs, recruitment incentives, tenure provisions, and differential pay. (Contains 71 notes.)