Teachers matter--and some matter more than others. That recognition has driven a tidal wave of controversial policy reforms over the past decade, rooted in new evaluation systems that link teachers' ratings and, in some cases, their pay and advancement to evidence of classroom practice and student learning. Two out of three U.S. states overhauled teacher evaluations between 2009 and 2015, supported by federal incentives such as Race to the Top and Teacher Incentive Fund grants, as well as No Child Left Behind Act waivers. What is the impact, so far, of these reforms? A closer look at one high-stakes evaluation system shows the positive consequences such systems can have for students. Since 2012, the authors have been studying IMPACT, a seminal effort by the District of Columbia Public Schools (DCPS) to link teacher retention and pay to their performance. Under IMPACT, the district sets detailed standards for high-quality instruction, conducts multiple observations, assesses individual performance based on evidence of student progress, and retains and rewards teachers based on annual ratings. Based on the authors' analyses, under IMPACT, DCPS has dramatically improved the quality of teaching in its schools--likely contributing to its status as the fastest-improving large urban school system in the United States as measured by the National Assessment of Educational Progress. DCPS dismissed the majority of very low performing teachers and replaced them with teachers whose students did better, especially in math. Other low-performing teachers were 50 percent more likely to leave their jobs voluntarily, and those who opted to stay improved significantly, on average, the following year. High-performing teachers improved their performance as well, especially those within reach of the significant financial incentive created by the system. Certainly, improvement was not universal, and some very good teachers decided to leave the district. Nonetheless, our analysis finds that improved teaching was common and that student achievement increased as a result. The DCPS story shows that well-designed and carefully implemented teacher evaluations can serve as an important district improvement strategy--so long as states and districts are also willing to make tough, performance-based decisions about teacher retention, development, and pay. [This article is based on research published in the March 2017 issue of "Educational Evaluation and Policy Analysis" and in the March 2015 issue of the "Journal of Policy Analysis and Management."]