There is growing evidence that well designed, targeted, theory-based behaviour change interventions can be effective in reducing the spread of HIV. Although each behaviour is unique, there are only a limited number of theoretical variables that serve as the determinants of any given behaviour. Understanding these variables and their role in behavioural prediction can guide the development of effective behaviour change interventions. This paper will describe and define these variables and show how they can be used in the development of behavioural interventions.