Dysfunctional behaviour of various forms can present challenges to nurses who care for older people. In particular, so-called ‘aggressive behaviour’ can present difficulties, especially among elderly people who have cognitive impairments, such as those produced by dementia. Many studies have been carried out to establish the prevalence of aggression, but reported rates vary considerably— from 18% to 65% (Ballard et al. 2000). One factor producing such a wide variation in incidence is that there is no consistent definition of what ‘aggression’ really is.