Many occupations recognize employees years of experience as a relevant factor in human resource policies. In education, teacher experience is a cornerstone of traditional single-salary schedules; it drives teacher transfer policies that prioritize seniority; and it is commonly considered a major source of inequity across schools and, therefore, a target for redistribution. The underlying assumption is that experience promotes effectiveness. But is this really the case? Do students attain higher levels of achievement when taught by more experienced teachers? Over 40 years of teacher productivity research suggests that the simple assumption that more is better requires greater nuance; experience effects are complex and depend on a number of factors. Recent evidence from CALDER studies provides new insight into the effects of teacher experience.