Public school teachers in the United States are famously difficult to dismiss. The reason is simple: after three years on the job, most receive tenure--after a brief and subjective evaluation process in which few receive negative ratings. In recent years, some school districts have experimented with changes in tenure rules. They seek the power to remove ineffective teachers and, in some jurisdictions, to reevaluate teachers throughout their careers. A keystone of this reform movement is the replacement of subjective evaluation with quantifiable measures of each teacher's effectiveness. The quantitative method is known as value-added modeling (VAM), a statistical analysis of student scores that seeks to identify how much an individual teacher contributes to a pupil's progress over the years. The use of VAM in teacher evaluations is growing, but the method remains extremely controversial. Critics often claim that it does not and cannot measure actual teacher quality. This paper addresses that claim. Part I analyzes data from Florida public schools to show that a VAM score in a teacher's third year is a good predictor of that teacher's success in his or her fifth year. Having established that VAM is a useful predictive tool, Part II of the paper addresses the most effective ways that VAM can be used in tenure reform. (Contains 9 figures, 2 tables and 8 endnotes.)