This research synthesis examines how teacher effectiveness is currently measured. By evaluating the research on teacher effectiveness and the different instruments used to measure it, this research synthesis contributes to the discussion of appropriate rigor and relevance of measures for different purposes (i.e., formative vs. summative evaluation). The findings are presented along with related policy implications. In addition, the synthesis describes how various measures have been evaluated, explains why certain measures are most suitable for certain purposes (high-stakes evaluation vs. formative evaluation, for instance), and suggests how the results of the study might be used to inform the national conversation about teacher effectiveness. A comprehensive definition of the components and indicators that characterize effective teachers is provided, extending this definition beyond teachers' contribution to student achievement gains to include how teachers impact classrooms, schools, and their colleagues as well as how they contribute to other important outcomes for students. Through this synthesis, the National Comprehensive Center for Teacher Quality (TQ Center) hopes to provide some practical guidance in how best to evaluate teacher effectiveness. Appendices include: (1) Tools for Measuring Teacher Effectiveness; (2) Technical Consideration in Assessing Teacher Effectiveness; (3) Outcomes of Interest in Teacher Evaluation; and (4) Comprehensive List of Studies with Summaries. (Contains 1 table.)