The International Agency for Research on Cancer estimates that 25% of breast cancer cases worldwide are due to overweight/obesity and a sedentary lifestyle. The preponderance of epidemiologic studies indicates that women who engage in 3-4 hours per week of moderate to vigorous levels of exercise have a 30%-40% lower risk for breast cancer than sedentary women. Women who are overweight or obese have a 50%-250% greater risk for postmenopausal breast cancer. Alcohol use, even at moderate levels (two drinks per day) increases risk for both premenopausal and postmenopausal breast cancer. Certain dietary patterns, such as high fat, low vegetables/fruits, low fiber, and high simple carbohydrates, may increase risk, but definitive data are lacking. These lifestyle factors are likely associated with breast cancer etiology through hormonal mechanisms. The worldwide trends of increasing overweight and obesity and decreasing physical activity may lead to an increasing incidence of breast cancer unless other means of risk reduction counteract these effects. Thus, adoption of lifestyle changes by individuals and populations may have a large impact on the future incidence of this disease.