The 2007 Public Education Reform Amendment Act led to 39 percent of the principals in District of Columbia Public Schools (DCPS) being dismissed before the start of the 2008-09 school year, and additional principal exits over the next few years. We measure the impact of replacing these principals on schoolwide student achievement by measuring the changes in achievement that occurred when principals were replaced, and comparing these changes to achievement in comparison schools within DCPS that kept the same principal. We find that after a new principal's third year in a school, average schoolwide achievement increased by 4 percentile points (0.09 standard deviations) compared with how students in the school would have achieved had DCPS not replaced the previous principal. For students in grades 6 to 8, the gains were larger and statistically significant in both math and reading.