abitual physical activity is associated with a number of health benefits, particularly in relation to the prevention H of cardiovascular disease. Psychological research has made useful contributions to the understanding of physical activity as a pattern of behaviour. We describe the behavioural epidemiology of physical activity, exercise assessment issues, and Australian data on the population prevalence and recent trends in exercise participation. We present a selective commentary on studies of the determinants of exercise participation, with examples of studies likely to be of particular relevance to psychologists. Studies of potential deterrninants (demographic and biological characteristics; past and present activity patterns; self-motivation; knowledge, attitudes, and beliefs; environmental characteristics) show inconsistent patterns of findings. Making better use of representative population data on physical activity, studying the role of environmental influences on participation, understanding more clearly the stages of involvement in exercise, and clarifying the factors related to these different stages are potentially fruitful directions for research. Such research is needed to inform public health initiatives to promote higher levels of involvement in physical activity.Requests for reprints should be sent to Neville Owen,