It is well established that teacher quality makes a difference in student learning. Since the implementation of No Child Left Behind in 2002, staffing every classroom with a high-quality teacher has been an official national priority. That goal entails an implicit requirement to assess teacher and teaching quality more rigorously than has been the case in the past. Despite decades of research on how best to assess teacher performance, however, no consensus has evolved on any single assessment strategy or collection of strategies--indicating that the problem of designing adequate and appropriate assessment is inherently complex and controversial. Such complexity has not, however, prevented the Obama administration from encouraging policymakers to define "good" teachers as those who produce gains in student achievement, measured by gains in standardized test scores. Notwithstanding the federal enthusiasm for test scores, many researchers have warned against using a single measurement of any kind as the primary basis for such important personnel decisions as teacher retention, dismissal or pay. While there are important questions about what achievement scores can--and cannot--indicate about individual teachers, there is no question that placing excessive emphasis on test scores alone can have unintended and undesirable consequences that undermine the goal of developing an excellent teaching force. Given the experience to date with an overwhelming focus on student achievement scores as a basis for high-stakes decisions, policymakers would do well to pause and carefully examine the issues that make teacher assessment so complex before implementing an assessment plan. To facilitate such examination, this brief reviews credible research exploring: the feasibility of combining formative assessment (a basis for professional growth) and summative assessment (a basis for high-stakes decisions like dismissal); the various tools that might be used to gather evidence of teacher effectiveness; and the various stakeholders who might play a role in a teacher assessment system. It also offers a brief overview of successful exemplars. Based on the research reviewed, it is recommended that policymakers employ an assessment system that targets both continual improvement of the teaching staff and timely dismissal of teachers who cannot or will not improve. Steps toward that goal include that policymakers: (1) Be clear about the purposes of any assessment before selecting strategies. Where formative and summative assessment are to be combined, plan to address the challenges of dual-purpose systems; (2) Involve all key stakeholders in system design; (3) Rather than employing a single assessment tool, gather evidence from multiple sources. Combine strategies so that the weakness of any single tool is offset by the strengths of another; (4) Be sure that the criteria for assessing performance, artifacts or other factors are credible and are well understood by teachers and assessors; (5) Provide high-quality, ongoing training for assessors and routinely calibrate their efforts to ensure consistent application of criteria; (6) Look to high-quality research on existing tools and programs to inform the design of assessment systems; and (7) Commit sufficient resources to produce high-quality, productive assessment. Brief Summaries of Teacher Evaluation Methods is appended. Also contains a section of notes and references combined. (Contains 1 figure and 99 notes.)