Health-related behaviour is an important issue for providers for health care. Understanding and being able to predict and influence health behaviour are all essential if client co-operation and participation is to be obtained. Since nurses spend more time with patients than any other health care professional, they are in a position to influence health behaviour. Optimal interaction with patients and potential patients is the result of careful application of theory. The health-belief model offers an approach to understanding health-related behaviour. A clear understanding of the cause of behaviour is necessary in order to predict change. A clear understanding of cause is also necessary for determining methods to influence health behaviour. As a new model and one developed for the healthy, the model needs development and testing for applicability in understanding health and other behaviour. Nurses can make a significant contribution in this development by testing aspects of the model. This paper summarizes the model and assesses the model using specific criteria for a theory.