This brief considers the problem of using value-added scores to compare teachers who work in different schools. The author focuses on whether such comparisons can be regarded as fair, or, in statistical language, "unbiased." An unbiased measure does not systematically favor teachers because of the backgrounds of the students they are assigned to teach, nor does it favor teachers working in resource-rich classrooms or schools. A key caveat: A measure that is unbiased does not mean the measure is accurate. An unbiased measure could be imprecise--thus inaccurate--if, for example, it is based on a small sample of students or on a test with too few items. This brief focuses strictly on the bias that may arise when comparing the value-added scores of teachers who work in different schools. Raudenbush and Willms' linear model is appended.