Breast cancer is the most common cause of cancer death in women worldwide. Rates vary about fivefold around the world, but they are increasing in regions that until recently had low rates of disease. Despite the numerous uncertainties surrounding the etiology of breast cancer, intensive epidemiological, clinical, and genetic studies have identified a number of biological and social traits as risk factors associated with breast cancer. Principal among them is the evidence of BRCA1 and BRCA2 susceptibility genes, familial history of breast cancer, age, higher socioeconomic status, ionizing radiation, tallness in adult life, alcohol consumption, and a variety of hormone and metabolic factors. Among the hormonal influences, a relevant etiological function has been ascribed to unopposed exposure to elevated levels of estrogens and androgens. In addition, new epidemiologic evidence has indicated that among the metabolic factors, glucose metabolism, hyperinsulinemic insulin resistance, and insulin-like growth factor bioavailability may also play a role in breast cancer. These endocrine and metabolic factors may represent future targets for breast cancer prevention.