Teachers are the single most important in-school factor that affects student achievement. As a result, over the last decade state leaders have taken aim at increasing educator effectiveness, including requiring the implementation of new teacher evaluation systems. A growing body of research suggests that the expectations a teacher sets for an individual student can significantly affect the student's performance. Teacher expectations can, for example, be based on student characteristics such as race, ethnicity, and family income level, or indicators of past performance. These expectations can cause teachers to differentiate their behavior towards individual students, such that teachers set lower expectations for some students, provide briefer (or no) feedback on student errors--and less positive feedback after correct answers--and grant students less time to answer questions. All of these teacher behaviors, when repeated day in, day out, over the course of a year or multiple school years, can negatively impact student performance and ultimately perpetuate the achievement gaps that plague the American education system. This issue of "The Progress of Education Reform" provides a review of the research on the relationship between teacher expectations and student achievement. It also explores how policy can be used to improve the way schools are evaluating for, monitoring, and providing training to teachers on the potential negative effects of rigid teacher expectations. (Contains 6 endnotes.)