At the same time that NCLB has given states a mandate to staff their classrooms with "highly qualified teachers," the federal government is pushing a dangerously narrow definition of the knowledge and skills that today's teachers need. Over the last decade, policy makers and business leaders have come to realize what parents have always known - teachers make the most difference in student achievement. Thanks to new statistical and analytical methods used by a wide range of researchers, the evidence is mounting that teacher quality accounts for the lion's share of variance in student test scores. However, while consensus is growing among school reformers that teachers are the most important school-related determinant of student achievement, there is not much more than ephemeral agreement on what we mean by "teaching quality" or what steps we must take to see that every student has access to high-quality teachers. The research literature on teacher quality - taken as a whole - sends a strong message to policy makers and practitioners that teachers need to know their subject matter and how to teach it. The demands on our public schools clearly require all teachers to know a great deal about how humans learn and how to manage the complexity of the learning process. Today this means knowing how to manage classrooms, develop standards-based lessons, assess student work fairly and appropriately, work with special-needs students and English-language learners, and use technology to bring curriculum to life for the many students who lack motivation. These skills can be readily learned through effective teacher education, induction, and professional development experiences, and new research shows that teachers who are better prepared to meet such varied challenges are more likely to remain in teaching.