Breast cancer is a rare disease in young women, yet is the leading cause of cancer deaths in all ethnic groups in the United States and many parts of the world. The epidemiology for breast cancer in young women is reviewed, focusing on women under 40, prior to the recommended screening age. Specific age comparison groups used and results for young women vary in the literature, yet there are some common results. Young women have low incidence rates of breast cancer compared to older women. However, cancer incidence increases at a faster rate with increasing age in young women. Their cancers tend to be larger and higher grade with poorer prognostic characteristics, resulting in a higher risk of recurrence and death from breast cancer when compared to older women. Many of the usual risk factors for breast cancer in older women also increase risk in younger women including increasing age, Black race, family history, later age at first birth and menarche, radiation exposure and lack of physical activity. Risk factors that have specific relevance to young women include reproductive factors, history of induced abortion or miscarriage, oral contraceptive use, smoking, and radiation exposure, most specifically for treatment of Hodgkin Disease.