There is a widespread consensus among practitioners, researchers, and policy makers that current teacher evaluation systems in most school districts do little to help teachers improve or to support personnel decision making. For this reason, new approaches to teacher evaluation are being developed and tested. There is also a growing consensus that evidence of teachers' contributions to student learning should be a component of teacher evaluation systems, along with evidence about the quality of teachers' practice. "Value-added models" (VAMs) for examining gains in student test scores from one year to the next are promoted as tools to accomplish this goal. Policy makers can benefit from research about what these models can and cannot do, as well as from research about the effects of other approaches to teacher evaluation. This background paper addresses both of these important concerns. (Contains 2 figures, 2 tables and 29 endnotes.)