Using administrative longitudinal data from five states, we study how value-added measures of teacher performance are affected by changes in state standards and assessments. We first document the stability of teachers' value-added rankings during transitions to new standard and assessment regimes and compare our findings to stability during stable standard and assessment regimes. We also examine the predictive validity of value-added estimates during nontransition years over transition-year student achievement. In most cases we find that measures of teacher value added are similarly stable in transition years and nontransition years. Moreover, there is no evidence that the level of disadvantage of students taught disproportionately influences teacher rankings in transition years relative to stable years. In the states we study, student achievement in math can consistently be forecasted accurately--although not perfectly--using value-added estimates for teachers during stable standards and assessment regimes. There was somewhat less consistency in reading, because we find cases where test transitions significantly reduced forecasting accuracy.