States are redesigning their educator effectiveness systems to provide more information and more support to improve teaching. In the process, they increasingly look beyond the most basic and historically most common view of measuring student performance: how many students "passed" the State test in a given year. A number of States now require an objective measure of student growth to be part of teacher evaluations. States are using student growth measures to understand teacher effectiveness for good reasons. First, student learning is the most important expectation we set for schools, and nothing in a school impacts student learning more than effective teaching. Second, new data systems permit far better links between student outcomes (tests, graduation, postsecondary experiences) and specific schools and teachers. This facilitates assessment of and systemic learning about changes to policy and practice that might lead to improvements in the quality of teaching and public schools. Finally, traditional methods of evaluating teachers that typically do not include objective measures of teacher performance have in most state education agencies (SEAs) and local educational agencies (LEAs) provided inadequate information about teacher effectiveness. In particular, these methods tend to yield high ratings for almost all teachers, and consequently these ratings have little value in predicting either future teacher effectiveness or student achievement. As a result, they have yielded little information that can help teachers become more effective practitioners. This brief describes various approaches to measuring student growth and what research says about the extent to which student growth may be used as a measure of teacher performance.