Racial disparity in arresting behavior is not only a concern for people of color; it can delegitimize law enforcement, increase tension between police and citizens, and even increase crime. Efforts at police reform stall, in part because racial disparity in policing was previously unmeasurable. The authors present three new measures of racial disparity in arrest, measured across more than 13,000 agencies nationwide, allowing for reliable analysis of disparity across time and geographic space. They demonstrate that, between 1999 and 2015, while crime rates generally declined, racial disparity in arrest increased substantially. Where the average police agency in 1999 arrested 5.48 Blacks for every White, the 2015 average was 9.25 arrests, nearly twice that. The increase derives largely from disparity in juvenile arrests by urban municipal agencies.