One in five schools loses its principal each year. Despite the prevalence of principal turnover, little empirical research has examined its effects on school outcomes. Because principal turnover may occur in response to or contemporaneous with a downturn in student achievement, the effect of a turnover is confounded with unobserved school-level factors. We employ a novel identification strategy that blocks each potential source of endogeneity to isolate plausibly causal effects of within- and between-year principal turnover. Using eight years of North Carolina administrative data from 2009-2018, we find that principal turnover is associated with significant decreases in student achievement and increases in teacher turnover. These effects are similar whether the turnover occurs over the summer or during the school year.