Aggression and violence pose major problems for forensic mental health services, for the criminal justice system and for society in general. The social and economic costs of violence are immense. Within institutions such as hospitals and prisons, apart from causing injury, psychological harm and stress in patients and staff, violence can contribute to poor morale, staff turnover, job dissatisfaction and the elimination of a therapeutic climate in which patients or prisoners can be assisted to change and to improve their well-being. It has been estimated that violence in healthcare settings has direct costs of at least £69 million per annum in the United Kingdom (National Audit Office 2003; Gadon et al. 2006). Within Wales, which has a population of less than 3,000,000, the Wales Audit Office (2005) estimated the cost of violence to the NHS as £6.3 million between 2003 and 2004. Health care workers and prison officers, in particular, are at high risk of being assaulted in their workplace, compared to other groups of workers (Duhart 2001).