Principals have important management roles, including responsibilities for teachers, curricula and budgets. Schools change principals frequently; about 20% of public school principals in the United States leave their positions each year. Despite the significance of principals and the regularity of principal departures, little is known about how turnover affects schools. Using twelve years of administrative data from North Carolina public schools, this paper explores the relationship between principal turnover and student achievement. Principal departures follow a downturn in student performance. Achievement continues to fall in the two years following the installation of a new principal and then rises over the next three years. Five years after a new principal is installed, average academic performance is no different than it was five years before the new principal took over. Increases in student achievement following a principal transition may reflect mean reversion rather than a positive effect of principal turnover.