This paper estimates the impact of police presence on crime using a unique database that tracks the exact location of Dallas Police Department patrol cars throughout 2009. To address the concern that officer location is often driven by crime, my instrument exploits police responses to calls outside their allocated coverage beat. This variable provides a plausible shift in police presence within the abandoned beat that is driven by the police goal of minimizing response times. I find that a 10% decrease in police presence at that location results in a 7% increase in crime. This result sheds light on the black box of policing and crime and suggests that routine changes in police patrol can have a significant impact on criminal behavior.