Over the past 50 years numerous empirical studies have examined the effects that teachers have on student performance. The findings about teacher effects on student performance have been mixed and inconclusive. Although there is systematic evidence that teacher effectiveness varies considerably, teacher characteristics such as education, experience, and salary are not always associated with student performance. In addition, teacher characteristics such as experience and education explain only a small part of the variability in teacher effectiveness. Over the past 15 years value-added models have been used to estimate teacher effects. This entry reviews the literature on teacher effects and focuses on the magnitude, stability, and estimation of teacher effects over time. Suggestions about future research on teacher effects are offered.