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 consent. Understanding the antecedents of misconduct will help to develop interventions that reduce misconduct.