There is increasing epidemiologic evidence of an association between body mass index and energy expenditure and the risk of breast carcinoma. Women who are overweight or obese, especially women who gain weight throughout adulthood, are at an increased risk for developing breast carcinoma after menopause. Conversely, overweight women are at a reduced risk for developing breast carcinoma in the premenopausal years. The association between body mass index and breast carcinoma risk has been observed in women from several racial and ethnic backgrounds. Many studies have found an association between increased physical activity and reduced risk for breast cancer. Studies regarding physical activity and breast carcinoma risk have been conducted primarily with white women; therefore, the cross-racial/ethnic patterns with this risk factor are unknown. This article reviews data regarding the associations between body mass index, physical activity, and breast carcinoma risk and presents potential mechanisms for the observed associations, such as sex hormones, reproduction, growth hormones, insulin, and immune function. It outlines challenges in measuring physical activity and body mass index across populations. Finally, the current study discusses limitations of the available data and suggests future research priorities. Obesity and a sedentary lifestyle may be two important risk factors for breast carcinoma that can be modified and thus may have significant public health impact in women from various racial and ethnic backgrounds. Cancer 2000;88:1248–55. © 2000 American Cancer Society.