Today, policing in the United States is facing a crisis of legitimacy and calls for reform. This Element examines this crisis and describes the adverse effects of problematic police behavior on community members, police officers, and public safety. A critical analysis of past reform efforts is offered, including why they have had limited success in changing police operations, police culture, or styles of policing. The central thesis of this Element is that most police reforms have failed because we continue to use the wrong metrics to evaluate police performance. Cities have yet to systematically measure what matters to the public, namely how people are treated by the police. Hence, this Element proposes a new system of accountability using data from body-worn cameras and contact surveys to measure and incentivize procedural justice. Translating evidence into real organizational change should improve street-level policing, enhance police legitimacy, and improve public safety.