Epidemiologists have identified several risk factors for breast cancer, yet clinical advice to women to change these risk factors has been uncommon. Physical activity promises to be one modifiable risk factor through which women can reduce their risk for breast cancer. Clinicians can now advise women that reducing risk for breast cancer may be one additional reason to adopt an active lifestyle. There are still questions about the type and amount of exercise needed, the ages at which exercise should be done, and the interactions with other risk factors such as reproductive and menstrual history, diet, body mass, alcohol intake, genetics, and hormone therapy. Finding answers to these questions will require a research agenda focused on the biology of exercise and breast cancer.