Maintaining a safe environment for incarcerated individuals and staff is a core task of any prison service. Concerns about risk of continued criminal activity and violence pose challenges for prison management. This paper discusses perceived causes of violence and how prison managers approach these issues. This is based on interviews with 45 prison managers from all Dutch prisons. A key finding is that prison-based organised crime is associated with a serious threat of violence in men‘s prisons. Other concerns include psychiatric problems and the vulnerability of people convicted of sex offences.