Breast cancer is the most common invasive cancer in women worldwide and the leading cause of death in US women in mid-life. Treatment has adverse effects, adding to the importance of finding modifiable risk factors. At the invitation of Susan G. Komen for the Cure, we reviewed studies of breast cancer and environmental pollutants, diet (assessed prospectively), body size, and physical activity, and animal studies that identify chemicals as potential mammary carcinogens. Databases developed in the review include information on 216 chemicals that increased mammary gland tumors in animal studies and 450 epidemiologic studies (accessible at www.silentspring.org/sciencereview and www.komen.org/environment). Exposure to potential mammary carcinogens is widespread from chemicals found in consumer products, air and drinking water pollution, food, and women's workplaces. Epidemiologic studies have included only a small number of chemicals identified as mammary carcinogens or as hormone disruptors, which may have implications for breast cancer; however, evidence is emerging for associations between breast cancer and polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and organic solvents. Prospective diet studies have not revealed consistent associations with breast cancer. Improved exposure assessment methods will help advance future human studies of both diet and environmental pollutants. Studies of physical activity show that it is protective. In the same vein as evidence-based medicine, messages for patients, policymakers, and the public should support decision-making based on the strength of current evidence; such messages might address exposure reduction for some pollutants. Investments in research on environmental factors in breast cancer have potentially large public health benefits.