Over the last decade, policy and business leaders have come to know what parents have always known: teachers make the greatest difference to student achievement. With new statistical and analytical methods used by a wide range of researchers, evidence has been mounting that teacher quality can account for a large share of variance in student test scores. The evidence on the distribution of qualified and effective teachers is also clear--and the findings are not good. Teachers who have met the demanding standards of National Board Certification and those who have generated higher "value-added" student achievement gains are far less likely to teach economically disadvantaged and minority students. As a result, high-poverty schools are more likely to be beset with teaching vacancies in math and special education, and much more likely to staff classrooms with out-of-field, inexperienced and less-prepared teachers. This policy brief focuses on the best practices for preparation programs that ensure teachers are prepared to teach effectively, especially in high-needs schools. (Contains 3 tables.) [This paper was written with Alesha Daughtrey and Alan Wieder.]