Because of its increasing incidence, breast cancer is a significant burden for women worldwide. In industrialized countries, breast cancer is the second-leading cause of cancer-related deaths among women, and it is estimated that 1 in every 8 women will develop the disease during her lifetime. Sufficient evidence indicates that a number of genetic, environmental and lifestyle risk exposures during life may play important roles in the etiology of this disease. The purpose of this paper is to review some etiologic factors and underlying mechanisms in relation to breast cancer risk. Based on the published literature, there is sufficient evidence that some established factors are associated with breast cancer risk. Age, early age at menarche, late menopause, height, post-menopausal obesity, family history of breast cancer, ionizing radiation, oral contraceptives, hormonal replacement therapy, mammographic density, some gene mutations and clinical conditions, such as benign breast disease, are associated with an increased risk of breast cancer. The risk decreases with early childbearing, high parity and physical activity, and breastfeeding. Alcohol increases the risk, while caloric restriction may confer protection from breast cancer. Epidemiological evidence for other nutritional factors is insufficient. These results suggest that breast cancer is a multifactorial disease where genetic susceptibility, environment, nutrition and other lifestyle risk factors interact. Better identification of modifiable risk factors and risk reduction of breast cancer may allow implementation of useful strategies for prevention.