There seems to be a tendency to consider police crime as a result of bad practice, lack of resources or mismanagement, rather than acts of criminals. However, examples illustrate that criminal acts are intentionally carried out by police officers on duty. For example, a 30-year-old female police officer in Norway used her computer access in the police department to change the addresses of her personal enemies in public systems, which represents a serious kind of document crime. Unauthorised disclosure of confidential information occurred in two out of ten criminal police cases in Norwegian courts in 2007. Police crime does not only occur in countries such as Indonesia, Pakistan, Mexico and Nigeria. It is common in the UK, USA, Australia and Norway as well — although on a smaller scale. Police crime tends to be discovered when investigating police complaints. Police crime is defined as intentional crime committed by police officers on duty. Policing police crime is defined as enforcing law on potential and actual criminal employees in the police organisation. In days gone by we would have said, ‘Who will watch the watchman?’, or ‘Who is guarding the guardian?’ — now we say, ‘Who is policing the police?’. This paper addresses the following question: what kind of knowledge is needed for policing the police?