Breast cancer is one of the leading causes of cancer and cancer mortality among Canadian women. Based on the current incidence rates, the National Cancer Institute of Canada has estimated that one in 10 women will develop breast cancer during her lifetime. In an effort to control this disease, various prevention strategies have been proposed. One promising strategy involves the promotion of exercise in healthy women. It has been hypothesized that exercise may protect against breast cancer by influencing regularity of ovulatory cycles, by changing body fat composition, or by enhancing natural immunity. In this paper both the epidemiological and experimental evidence that speaks to a protective effect of exercise is reviewed. Although there are several biologically plausible mechanisms for the association, the existing epidemiological and experimental data are inconclusive due to the small number of studies and their methodological insufficiencies. However, given the enormous potential benefit of even a small protective effect of exercise, further studies designed to address the relationship between exercise and breast cancer risk are warranted.Key words: physical activity, mammary carcinogenesis, epidemiology