Classified boards are a major takeover defense for U.S. firms, but their impact on firm value remains ambiguous. This study uses the Hausman-Taylor instrumental variables model and a difference-in-difference-in-difference model to examine the relationship between classified boards and firm value. The findings suggest a negative and statistically significant association between classified boards and firm value. However, quantile regressions indicate that this negative association may be concentrated among the most valuable firms. The study also finds that classified boards may have a positive effect on firm value for less valuable firms. The analysis addresses limitations in previous empirical studies by controlling for unobserved heterogeneity and using panel data. The results suggest that classified boards may not always be detrimental to firm value, but they do appear to reduce firm value on average. The study contributes to the literature by providing a more nuanced understanding of the relationship between corporate governance and firm value.Classified boards are a major takeover defense for U.S. firms, but their impact on firm value remains ambiguous. This study uses the Hausman-Taylor instrumental variables model and a difference-in-difference-in-difference model to examine the relationship between classified boards and firm value. The findings suggest a negative and statistically significant association between classified boards and firm value. However, quantile regressions indicate that this negative association may be concentrated among the most valuable firms. The study also finds that classified boards may have a positive effect on firm value for less valuable firms. The analysis addresses limitations in previous empirical studies by controlling for unobserved heterogeneity and using panel data. The results suggest that classified boards may not always be detrimental to firm value, but they do appear to reduce firm value on average. The study contributes to the literature by providing a more nuanced understanding of the relationship between corporate governance and firm value.