Two decades of research have documented what now is common knowledge: no in-school factor matters more to students' educational experiences and outcomes than the effectiveness of their teacher. As a result, the national airspace is increasingly crowded with proposed reforms and initiatives designed to boost teacher performance, including new evaluation systems and increased accountability. This activity was crystallized by the U.S. Department of Education's 2010 Race to the Top competition, which emphasized the development of evaluation systems that involve measures of student growth. PACER's goal is to inform state education policy discussions using rigorous, objective research. This issue brief presents a summary of research and emerging practices in other states and districts designed to provide state lawmakers, legislative staff, State Board of Education members, Department of Education officials, and other key stakeholders information they need to ensure the effectiveness of their reform efforts. The roughly two dozen works cited include newspaper coverage; government documents (e.g., federal agency guidance); research syntheses from non-partisan sources such as the Consortium for Policy Research in Education and the National Comprehensive Center for Teacher Quality; original research by Research for Action (RFA) scholars; and rigorous, peer-reviewed works from some of the nation's most respected education researchers. A list of works cited and notes is provided.