Breast cancer is mainly a postmenopausal disease, but in younger women breast tumors often exhibit more aggressive features and worse prognosis. Furthermore, high-risk and low-risk tumors present different age distributions suggesting that breast cancer comprises a mixture of two different disease processes. In agreement with this hypothesis, breast cancer presents different epidemiologic traits in pre- and postmenopausal women. Regarding racial distribution, incidence is higher in black women at younger ages in US, while the reverse is true among women older than 50 years. Genetic predisposition is a stronger risk factor in young women. On the contrary, nulliparity and obesity decrease the risk of early-onset breast cancers while are associated with higher incidence in older women. Epidemiologic data related with the hormonal exposure in utero suggest that the effect is stronger in early breast cancers. In most developed countries, breast cancer has shown an upward trend until recent years in postmenopausal women, while incidence rates in younger women have been stable. However, Spain is an exception to this rule: Spanish women younger than 45 years of age have registered a steady increase of breast cancer that may be related with the remarkable lifestyle changes experienced by women born in the second half of the twentieth century.