Epidemiologic research continues to confirm a strong inverse association between physical activity and breast cancer risk. New studies suggest that activity during adolescence is most important for premenopausal breast cancer. Long-term activity patterns are clearly important for postmenopausal breast cancer, although recent activity often highly correlates with long-term activity, accounting for its influence on risk. Whether physical activity has greater influence on estrogen receptor- or progesterone receptor-negative breast cancers or affects receptor-negative and receptor-positive tumors equally remains a question. Studies have established that lower sex hormone levels likely explain lower breast cancer risk in physically active women. Overall, the evidence regarding physical activity and breast cancer risk highlights the importance for young women to adopt a physically active lifestyle that is maintained throughout life and for medical practitioners to counsel patients and prescribe exercise for its critically important health benefits, which include reducing a woman's breast cancer risk.