Controversial police use of force incidents have spurred protests across the nation and calls for reform. Body-worn cameras (BWCs) have received extensive attention as a potential key solution. I conduct the first nationwide study of the effects of BWCs in more than 1,000 agencies. I identify the causal effects by using idiosyncratic variation in adoption timing attributable to administrative hurdles and the lengthy process to the eventual adoption at different agencies. This empirical strategy addresses limitations of previous studies that evaluated BWCs within a single agency; in a single-agency setting, the control group officers are also indirectly affected by BWCs because they interact with the treatment group officers (spillover), and agencies that give researchers access may fundamentally differ from other agencies (site-selection bias). Overcoming these limitations, my multi-agency study finds that BWCs have led to a substantial drop in the use of force, both among whites and minorities. Nationwide, they reduce police-involved homicides by 43%. Surprisingly, I do not find evidence that BWCs are associated with de-policing. Examining social media usage data from Twitter as well as data on the Black Lives Matter (BLM) movement, I find that after BWC adoption, public opinion toward the police improves. These findings imply that BWCs can be an important tool for improving police accountability without sacrificing policing capabilities.