Reforming teacher tenure is an idea that appears to be gaining traction with the underlying assumption being that one can infer to a reasonable degree how well a teacher will perform over her career based on estimates of her early-career effectiveness. Here we explore the potential for using value-added models to estimate performance and inform tenure decisions. We find little evidence that the variation of teacher effects change over teacher careers, but strong evidence that prior year estimates of job performance predict student achievement, even when there is a multi-year lag between the two. (Contains 46 footnotes, 5 tables and 7 figures.)