Under laws providing for equal access to quality education, most American students with disabilities are taught largely in general education classrooms. And along with all other students, they are included in standards-based reforms--policies that hold their schools and teachers accountable for what they do or do not learn. In more and more cases, policies require that a teacher be judged partly by his value-added--a measure of what he contributes to student learning as determined by student scores on standardized tests. States and districts, of course, want to make sure they have accurate measures of these contributions. But students with disabilities pose several challenges for calculating value-added. They tend to score low--often very low--on regular state assessments, and most receive accommodations, such as extra time. The result can be scores that are unreliable or not comparable to those of other students or across years. In other cases, students with disabilities have test scores that cannot be used to calculate value-added because they take alternative assessments. At the same time, many students with disabilities are taught by multiple teachers; they may be taught by two teachers in the same classroom or by different teachers in separate general and special education classrooms. Students with disabilities also often receive help from aides and special services. Disentangling the contribution of each teacher from these other factors may be difficult. In this brief, the authors discuss the challenges of using value-added to evaluate teachers of students with disabilities. They consider the limited empirical research on the potential for systematic errors in value-added for these teachers, either because the models do not adequately account for the likely achievement growth of their students, or because they do not account for teachers being more or less effective for students with disabilities than they are for other students. They also consider the comparability of value-added for special education teachers and the value-added for other teachers.