This paper provides the first population-based evidence on how much standardized test scores vary among public school districts within each state and what factors explain that variation. Using over 200 million standardized test records in math and ELA for grades 3 through 8 from every U.S. public school district during the 2008-09 to 2012-13 school years, we estimate intraclass correlations (ICCs) as a measure of between-district variation. We characterize the variation in the ICCs across states, as well as the patterns in the ICCs over subjects, grades and cohorts. Further, we investigate the relationship between the ICCs and measures of racial and socioeconomic segregation. We find that between-district variation is greatest, on average, in states with high levels of both white-black and economic segregation among school districts.