Educational policymakers struggle to find ways to improve the quality of the teacher workforce. The early career period represents a unique opportunity to identify struggling teachers, examine the likelihood of future improvement, and make strategic pre-tenure investments in improvement as well as dismissals to increase teaching quality. To date, only a little is known about the dynamics of teacher performance in the first five years. This paper asks how much teachers vary in performance improvement during their first five years of teaching and to what extent initial job performance predicts later performance. We find that, on average, initial performance is quite predictive of future performance, far more so than typically measured teacher characteristics. Predictions are particularly powerful at the extremes. We employ these predictions to explore the likelihood of personnel actions that inappropriately distinguish performance when such predictions are mistaken as well as the much less discussed costs of failure to distinguish performance when meaningful differences exist. The results have important consequences for improving the quality of the teacher workforce.