We estimate causal peer effects in police misconduct using data from about 35,000 officers and staff from London's Metropolitan Police Service for the period 2011-2014. We use instrumental variable techniques and exploit the variation in peer misconduct that results when officers switch peer groups. We find that a 10% increase in prior peer misconduct increases an officer's later misconduct by 8%. As the police are empowered to enforce the law and protect individual liberties, integrity and fairness in policing are essential for establishing and maintaining legitimacy and public consent1-5. Understanding the antecedents of misconduct will help to develop interventions that reduce misconduct.