In this paper we report on work estimating the stability of value-added estimates of teacher effects, an important area of investigation given public interest in workforce policies that implicitly assume effectiveness is a stable attribute within teachers. The results strongly reject the hypothesis that teacher performance is completely stable within teachers over long periods of time, but estimates suggest that a component of performance appears to persist within teachers, even over a ten-year panel. We also find that little of the changes in teacher effectiveness estimates within teachers can be explained by observable characteristics. The Influence of Bias is appended. (Contains 6 tables, 4 figures and 41 footnotes.)