Teacher quality matters. In fact, it is the most important school-related factor influencing student achievement. Moreover, teacher compensation represents a significant public investment: in 2002 alone, the United States invested $192 billion in teacher pay and benefits. Given the size of this investment, there is remarkably little research to guide such critical decisions as whom to hire, retain, and promote. In the absence of a strong, robust, and deep body of research, the debate in this field is largely ideological.