Tenure is intended to protect teachers with demonstrated teaching skills against arbitrary or capricious dismissal. Critics of typical tenure processes argue that tenure assessments are superficial and rarely discern whether teachers in fact have the requisite teaching skills. A recent reform of the tenure process in New York City provides an unusual opportunity to learn about the role of tenure in teachers' career outcomes. We find the reform led to many fewer teachers receiving tenure. Those not receiving tenure typically had their probationary periods extended to allow them an opportunity to demonstrate teaching effectiveness. These "extended" teachers were much more likely to leave their schools and be replaced by a teacher who was judged to be more effective.