Transitions to a new principal are common, especially within urban public schools, and potentially highly disruptive to a school's culture and operations. We use longitudinal data from New York City to investigate if the effect of principal transitions differs by whether the incoming principal was hired externally or promoted from within the school. We take advantage of variation in the timing of principal transitions within an event-study approach to estimate the causal effect of principal changes. Changing principals has an immediate negative effect on student test scores that is sustained over several years regardless of whether hired internally or externally. However, externally hired principals lead to an increase in teacher turnover and a decline in perceptions of the school's learning environment, whereas transitions to an internally promoted principal have no such effects. This pattern of results raises important questions about leadership transitions and the nature of principal effects on school quality.